Insects
A wide variety of insects, spiders, snails, slugs and other invertebrates are commonly found in specialty crops. While some are damaging to plants, many are not. Some may even be beneficial, such as pollinators (e.g. bees) or those that act as natural predators of pest insects (e.g. ladybeetles). The simple presence of an invertebrate on your crop is not necessarily a reason to undertake control measures. Ensure it is actually damaging your crop before trying to remove it. Additionally, remember that any control measure used against a pest insect could also be harmful to its natural enemies or to pollinators.
Some damage is distinctive, allowing identification of the insect that caused it without even seeing the pest at work. A hand lens (e.g. 10x-20X) can be helpful to see some tiny pests such as mites. Other pests develop within the plant tissues, concealed from view. Some common types of insect pests from each category are listed below. More detailed information on specific pests is listed under each crop.
DEFOLIATORS AND CHEWING INVERTEBRATES
Defoliating and chewing invertebrates eat plant tissue, including leaves, flowers, buds, stems, fruit and seeds. They may feed singly or in dense colonies and their feeding habits vary. They chew or scrape plant parts. Defoliators attack leaves and stems, consuming leaf edges, interveinal tissue or entire leaves. Some remain hidden, feeding inside the leaf and creating extensive mines visible as blotches or trails. Either way, photosynthesis is affected, and the plant is deprived of food. While some defoliators can be seen, many others hide within a web, folded leaf or portable shelter. Other species of chewing insects feed directly on fruits, nuts, vegetables or seeds, and are often more damaging then those that feed on foliage alone.
A. Caterpillars
Caterpillars are the immature, or larval, stages of moths and butterflies. They may feed on foliage, fruits or nuts; either in colonies or individually. Some feed on the surface of the leaf or fruit, while others chew their way inside and may be difficult to detect. The adult stages do not have chewing mouthparts; many do not feed at all while others feed only on nectar or other sources of sugar.
Caterpillars have three pairs of true legs on their thorax, located just behind the head. Up to 5 pairs of fleshy "prolegs" with hooks on the bottom are located further back on their abdomens. The presence of these “prolegs” aids in distinguishing caterpillars from the larval stages of other insects, such as beetles and maggots. This group also includes loopers (inchworms) that have only two or three pairs of prolegs. Some caterpillars make shelters for themselves by folding or rolling leaves around themselves, holding these together with webbing. Caterpillars making this type of shelter are known as leafrollers. Others form large colonies of insects living on or in extensive webs or tents. These shelters help protect them from predators and parasites.
Cutworms - Cutworms are members of a group of caterpillar species that often cause a distinct type of damage early in the growing season. They attack stems of seedling plants, clipping them near the soil level and causing plants to fall over. Large cutworms can destroy several seedlings, leaving patches of dead plants. Some species will also attack foliage or other plant parts later in the season. Cutworms feed only at night and remain hidden on or just below the soil surface during the day. They are most active in May and June. Cutworms are variable in appearance but are usually dull grey to brown with darker markings, and are often hairless and greasy looking. They tend to curl into a "C" when disturbed.
B. Beetles
Beetles feed as larvae and adults, though the type and location of plant parts consumed can vary with their life stage. They can completely devour, skeletonize (leave only the veins behind) or mine the leaves. Those attacking fruit may cause direct damage to fruit and fruiting vegetables, or they may feed on damaged areas caused by other animals.
Beetles are highly variable in size and colour, but are readily identified by most people by their hardened outer wings, called elytra, which when the insect is not flying are held flat against the abdomen, meeting in a straight line down the back. These protect the hindwings, which are used for flying and at rest are folded underneath the elytra. All beetles have chewing mouthparts, which can be used for feeding either on plants or other invertebrates, depending on the species. Some of the more common beetles attacking Ontario specialty crops are listed below.
Japanese Beetles - Japanese beetles were introduced into North America on nursery stock from Japan in 1913. They have become seasonal pests of many crops. Adults beetles are 10-13 mm long, metallic green to greenish-bronze, with coppery red wings and small white tufts on the sides and tip of their abdomen. Adults feed on the upper surface of foliage, chewing the tissue between the veins, creating a skeletonized appearance. Beetles usually feed in groups starting at the top of the plant and moving downward. They can fly as far as 8 km, but usually only make short flights as they move around to feed and lay eggs. Japanese beetles produce aggregation pheromones that attract more males and females to feed and mate. Odours emitted from damaged plants may also attract more beetles.
Adults begin emerging in late-June to early-July and they will live for 30 to 45 days. Females lay 40 to 60 eggs in the soil during July and August, and the larvae hatch in 10 to 14 days. Larvae are "C" shaped, white to cream-coloured grubs with brown heads. The grubs feed on organic matter and grass roots prior to moving into overwintering sites.
Japanese beetle traps are available at garden centers. Although the pheromone and floral lures sold with the traps are very effective and can attract many beetles each day, research has shown that the traps attract more beetles than are caught. As a result, susceptible plants in the vicinity of the trap are likely to suffer more damage than if no traps were used.
Flea beetles – Flea beetles are small (2-3 mm, ½-1/8 in.), shiny black or brown beetles. The adults are active and jump when disturbed. Feeding damage consists of numerous “shot-holes”, 1-5 mm (1/25 – 1/5 in.) in diameter. Plants are usually not killed, but growth is delayed, however they are capable of destroying enough foliage on seedlings to kill the plant. Serious damage occurs mostly on young plants. They attack numerous crops, including potato, tomato, pepper, eggplant, cabbage, cauliflower, broccoli, oriental vegetables, Brussels sprouts, radish, rutabaga, corn and sweet potatoes. There are several species of flea beetles that attack crops in Ontario, including crucifer flea beetles, striped flea beetles, potato flea beetles and corn flea beetles.
Adult flea beetles overwinter in leaf litter. They emerge and begin feeding on young plants in mid-May. Eggs are deposited near the roots of host plants throughout the spring and early summer. Larvae develop on the roots. In late July, adults emerge from the soil, feed and then seek hibernation sites in the fall. Cold and/or wet spring and early summer temperatures tend to keep damage pressure of this pest low.
Colorado Potato Beetles - Colorado potato beetles, which attack potato, eggplant, pepper tomato, and related species are about 10 mm long. Adults are orange and yellow with 10 black stripes on the wings Larvae are red-to-orange with two rows of black spots along their sides. The eggs are orange and are laid on the underside of the leaf. Adults and larvae feed on the foliage, chewing irregular holes on leaves or occasionally stems. Feeding on fruit surfaces may also occur. High populations can completely defoliate plants.
Cucumber beetles - The cucumber beetle is a major pest of all cucurbits. Beetles attack all parts of both young and mature plants. Young plants are most susceptible to yield losses due to direct feeding on the cotyledons and developing leaves. High populations of beetles can completely skeletonise the plant. The cucumber beetle is the primary vector of bacterial wilt disease. Once a plant is infected, there is no control for bacterial wilt. Effective control of the beetle population is the only method to prevent crop losses due to bacterial wilt infections.
Striped Cucumber Beetles are the most common species found in Ontario cucurbit fields. They are approximately 6- 7 mm (1/4- 2/7 in.) long, with 3 black stripes on yellow wing pads. The stripes of the cucumber beetle extend the entire length of the wing pad. The lower portions of the legs are black, giving the beetles the appearance of wearing knee-socks. The Spotted Cucumber Beetle, also known as the Southern Corn Rootworm, is slightly larger than the striped cucumber beetle. It has yellow wing pads with black spots.
The first generation of adults emerges in mid-June to early July, precisely as many direct-seeded cucurbits emerge. They may be present in the field throughout the summer. However, early populations are generally the most damaging. Late-season feeding on the fruit may reduce marketable yields.
C. Leafminers
Leafminers are the immature stage of some small insects, typically flies (maggots) or caterpillars, which develop within leaf tissues. Leafminers lay their eggs into leaves so that the larvae feed between the leaf surfaces. As they grow, they move through the leaf's interior, feeding on the sap and tissues and leaving characteristic tunnels which are easy to identify. Tunnels can be straight, serpentine or irregular. Feeding punctures and egg-laying punctures may also cause serious damage in some cases. The larvae of leafminers usually mature in the leaf and then drop to the ground to pupate. There may be several generations per year.
There are several species of leafminers which may attack vegetables, ornamentals and fruit in North America. Identification of many species may be difficult since they are very similar in appearance and behaviour.
Leek Moth - The leek moth, or onion leafminer, is an invasive pest from Europe that damages species in the allium family. It was first identified in Eastern Ontario in 1993. The larvae tunnel mines and create pinholes in leaf tissue and scapes, sometimes causing leaf distortion. They occasionally attack the bulb and stem. Damage to garlic cloves may predispose them to diseases. Larvae are yellowish-green with small, greyish spots and a pale brown head. The adult is a small, reddish-brown moth with a white triangular mark on the middle of the folded wings. Adults emerge in the spring when temperatures reach 9.5°C, mate and lay eggs. There are three generations per season, with damage typically increasing as the season progresses.
D. Fruit- and Seed-Feeding Maggots
Maggots are the larval stages of flies. They are typically small, white and legless. While some maggot pests attack roots (see soil insects), others damage aboveground parts. Some species attack seeds and germinating seedlings (e.g. the seedcorn maggot) while others damage fruits above ground (e.g. the apple maggot).
Spotted Wind Drosophilla - Spotted wing drosophila (SWD) is an invasive vinegar fly from Asia that can cause extensive damage to soft-skinned fruits before harvest. First detected in North America in 2008, this pest has spread quickly. SWD has been found in Ontario, British Columbia and Nova Scotia, as well as many American states and in Europe (e.g. Spain, Italy and France). SWD is different from other vinegar flies because it lays eggs in healthy ripening fruit, rather than overripe or damaged fruit. This means that SWD larvae may be in fruit at harvest, and consumers are likely to notice larvae in fruit when eaten, cooked or frozen. Infested fruit breaks down quickly, and is especially leaky, reducing the shelf life. Blackberries and raspberries are preferred hosts, followed by blueberries, cherries, strawberries and a wide range of other soft-skinned fruit.
The SWD adults and maggots are very similar in appearance to the common vinegar fly normally associated with over-ripe, decaying or damaged fruit. They are not closely related to the much larger fruit fly maggots found in fruit, such as the apple maggot, the blueberry maggot, and the cherry fruit fly. Adults are small (2-3 mm), red-eyed flies with a pale yellow or brown abdomen marked by dark brown complete unbroken bands or stripes. Males have a distinct black www.ontario.ca/spottedwing
Swede Midge - The swede midge is a gall fly native to Europe and Asia that was first found on Brassica plants in Ontario in 2000, and is now widely distributed in Ontario. It is a tiny, light-brown fly (1.5-2 mm), difficult to distinguish from other midge species in Ontario. The larvae are small (0.3-3 mm long), initially translucent maggots that become yellow-white when mature. They typically feed in clusters on young tissue, near growing points. Damaged seedlings often appear twisted and may have a noticeable brown scar or a gall at the growing point. Later feeding injury results in twisted and distorted heads. Damage can be confused with other problems such as nutrient deficiency, heat stress or frost damage. The first generation of swede midge adults appear from mid-May through early June. There are 4-5 overlapping generations in Ontario.
The swede midge is very difficult to control. Swede midge is commonly spread to new areas by the movement of transplants from infested areas. Be confident about the source of your transplants and do not bring infested plant material into clean areas. Pupae may remain in the soil for 2 years, so avoid growing any crucifer crops in infested soil for 2-3 years. There is considerable variability among plant varieties in susceptibility to swede midge.
Seedcorn Maggot – The seedcorn maggot, the larval stage of the fly Delia platura, is a pest of many vegetable crops. Corn, squash, cucumber, radish, beans, peas, and beets are affected. Seed-corn maggots are dirty-coloured yellowish-white larvae and about 8-10 mm long with long, sharply pointed heads. They burrow into the seed where they can be found under the seed coat. The seed usually fails to germinate or sprout, or growth is weak and spindly. The adult flies lay their eggs in May in recently manured soil and rotting vegetation. Larvae hatch in 7- 10 days and remain in the field for 1- 3 weeks feeding on residue, seeds and young seedlings. Larvae may be active at soil temperatures as low as 10°C (50°F). There are 3- 6 generations per year. Once the plants grow past the seedling stage, they are no longer susceptible to seedcorn maggot injury. Seedcorn maggots are commonly found during cool, wet springs when germination is delayed and early crop growth is slow.
E. Slugs and Snails
Slugs and snails are molluscs and so are more closely related to clams than insects. Both are soft-bodied, slimy and legless, however slugs do not have an obvious external spiral shell. They have rasping mouthparts that they use to eat holes in leaves, roots and other parts of many different plant species. They generally hide during the day in dark, moist places and feed at night, leaving behind glistening, sticky slime trails, which serve as evidence of their activity. Slugs are often more damaging than snails. Damp, rainy weather and mulches favour slugs.
SUCKING INSECTS
Sucking insects weaken plants by removing water, nutrients and other material. Some also inject secretions that injure or kill plant cells. Some sucking insects can transmit certain plant diseases. Because leaves are not chewed or torn, infested plants may suffer severe injury before symptoms appear. Signs of damage include mottling and fading leaf colour, curling and twisting leaves, wilting of foliage and tender shoots, and malformed flowers.
A. Aphids
Aphids (plant lice) are small, soft bodied, pear-shaped insects of varying colour (often green, red or black), They can be distinguished from other similar insects by the presence of two projections near the tip of their abodomen, called cornicles or tailpipes. Aphids have long, thin straw-like mouthparts that they use to pierce plant tissue and suck out fluids. As they feed, they secrete a sweet sticky substance on which black sooty mould may grow. Aphids often form dense colonies, clustering on new growth and young shoots, on the underside of leaves, along the stem or on petioles. Aphid populations can increase very quickly under favourable conditions.
B. Leafhoppers
Leafhoppers are small (approximately 3-5 mm long), wedge-shaped active insects, usually green, yellow or striped. They can be distinguished from aphids by the lack of cornicles and their tendency to run, hop or fly away when disturbed. They are related to aphids, and also possess piercing-sucking mouthparts which they use to feed on the undersides of leaves, causing stippling or bleaching; small, white dots appear on the upper surface of leaves. Some leafhoppers secrete toxins that can kill tissue, leading to leaf margins turning black (necrosis). The cast skins of the nymphs often remain attached to the lower surface of leaves.
Potato leafhopper – The potato leafhopper (PLH) is a common pest of many vegetable and fruit crops in Ontario. PLH do not overwinter in Ontario. Each spring adults are carried by wind currents from southern Gulf States and across the Great Lakes into Ontario. The first adults arrive as early as mid May and continue to arrive well into June. Nymphal development takes from 8-25 days, depending on temperature. Three or four overlapping generations are produced each year and remain active until killed by a hard frost. In hot, dry summer weather, leafhopper populations can build to large numbers.
Adults have a very wide host range, but plant maturity affects suitability and not all support development of immature stages. One of the first places they show up is in deciduous hosts, before moving into annual crops, various weeds, vegetables, apples, strawberries and grapes. Preferred hosts include alfalfa, beans, and potatoes. The removal of a food source (such as alfalfa) usually causes adults to migrate to other crops.
PLH adults are tiny (3 mm), wedge-shaped, and pale yellow-green in colour. They are highly mobile and are easily disturbed when you approach infested foliage. Nymphs are similar in appearance but lack wings. When nymphs moult, they leave behind cast skins, which can be seen during scouting activities. Older nymphs develop "wing pads" that distinguish them from the fully winged adults. Nymphs walk sideways or backwards, and rapidly move to the underside of the leaf if disturbed.
Leafhoppers suck plant sap from foliage causing white stippling. PLH are particularly damaging because they inject a toxin into the plant while feeding, blocking vascular system flow and preventing normal movement of water and nutrients to the affected area of the plant. Foliage may become stunted and deformed. Leaves turn pale green and curl downward at the margins. Leaf margins eventually turn brittle and brown, a condition known as "hopperburn". Water stress makes plants more susceptible to injury. Thresholds have not been developed for most crops, but it takes only 2 or 3 PLH to cause a leaf to curl. Yield losses may occur before the development of symptoms. Monitor for leafhoppers using sweep nets or visual examination of leaves.
C. Mites
Mites are relatives of spiders. Mites have either 2 or 4 pairs of legs and two main body regions that may appear fused; in contrast, insects have three pairs of true legs and three body regions. Most are extremely small, and are not readily seen with the naked eye. One of the most common mite pests in crops are spider mites, which produce webbing to protect themselves from their natural enemies and from being dislodged. Mites reproduce rapidly during hot dry weather, and can build up to damaging levels very quickly. They insert their mouthparts into the tissue of a variety of plants, causing speckling, bleaching or bronzing of the foliage.
D. Plant Bugs
Plant bugs belong to a group of insects known as true bugs. Adults of most species can be identified by the "X" shape made by their wings when folded on their backs, and by the distinctive triangle located behind their heads. Plant bugs are related to aphids and also use strawlike mouthparts to suck sap from plant tissue. Both adults and nymphs cause damage, often injuring the young, developing shoots and leaves. Symptoms of damage include stippled leaves, deformed leaves and stunted shoots. On fruits and vegetables, they often cause depressions in the tissues around where they feed.
Tarnished Plant Bug - The tarnished plant bug, Lygus lineolaris, (TPB) is one of the most common stinging insects in Ontario. Tarnished plant bugs have a large host range. They are known to attack over 270 different plant species. Their ability to feed on a wide range of hosts and have multiple generations per year make them an extraordinary and resilient pest. Certain weeds are a preferred habitat for this insect. It can commonly be found on members of the mint family, chickweed, pigweed and alfalfa.
Adult bugs have a characteristic shiny yellow to brown-black appearance with a distinctive triangular pattern on their backs. They are about 5 mm (3/16 in.) long. Nymphs are green in colour and resemble aphids. Nymphs progress through five instars before moulting to adults. The final three instars have wing pads. There are three to five generations of tarnished plant bug per year.
They are present throughout the growing season, injecting toxic saliva into the plants, causing spotting and decay of flowers and shoot tips, thus allowing diseases to enter through the wounds. They also feed on fruit, causing indentations or holes, and corkiness of the flesh, where the fruit is stung by the piercing-sucking mouthparts. The fruit may become misshapen or may crack at the wound.
Four-lined Plant Bug - The four-lined plant bug, Poecilocapus lineatus, is a common pest with a broad host range that includes numerous fruits, vegetables, ginseng and ornamental plants. For many crops, the damage is mostly cosmetic, however this insect seems to have a particular affinity for members of the mint family (including lavender, sage, oregano, mint, basil, marjoram, savory and catnip) and can in some cases cause economic damage to herbs. Four-lined plant bugs are active from early May through late June or early July.
Adult four-lined plant bugs are 7.0-7.5 mm in length and easily identified by the four distinctive black bands running down the back of their yellow- to green-coloured hind wing covers. The antennae are black and the legs are black and greenish yellow. Nymphs are bright red or orange, with black dots on their back, and can be mistaken for the nymphs of several other plant bug species. Newly hatched nymphs are wingless but, as they molt to adulthood, develop dark wingpads which run partway down their abdomen.
Both adults and nymphs have sucking mouthparts which they use to pierce the tissue of young foliage and suck the fluids out, leaving "windows" consisting of only the outer plant skin. They also release toxins into the plant, resulting in whitish to dark spots on plant leaves. These can coalesce into large patches of discoloured tissue and can result in distortion, curling, browning and death of leaves. The spots produced by four-lined plant bug feeding resemble the symptoms of fungal leaf spot diseases, and dead tissue can drop out in a similar fashion to plant disease.
E. Stink Bugs
The term "stink bugs" refers to a group of insects with a shield-shaped body that produce disagreeable defensive odours that are released when disturbed. Most species are plant feeders, some being significant agricultural pests, while others are considered beneficial insects that feed on pest insects. Plant feeding (pest) stink bugs feed on fruit, pods and seeds. The piercing-sucking mouthparts of the adults and nymphs cause cloudy yellow blotches just below the skin of the fruit.
Adults are brown or green and shield-shaped, about 1.3- 1.9 cm (½-3/4 in.) long. Nymphs are smaller, rounded, and wingless. They may be a different colour than the adult.
Some stink bugs feed on insect pests. You can distinguish the plant feeding (pest) stink bugs from the predatory (beneficial) stink bugs by the proboscis (beak). Predatory stink bugs have a wide proboscis for attacking other insects, while plant-feeding stink bugs have a narrow, needle-like proboscis for probing plants.
Stink bugs that feed on Ontario crops include the southern green stink bug (Nezara viridula), the green stink bug (Acrosternum hilare) and the brown stink bug (Euschistus servus). The brown marmorated stink bug (Halyomorpha halys) is an emerging stink bug pest in Ontario.
Brown Marmorated Stink Bug - The brown marmorated stink bug (BMSB), Halyomorpha halys (Stål), is an invasive alien insect native to Japan, Korea, Taiwan and China. The first official North American detection of this pest occurred in Pennsylvania (PA) in 2001, but it has now spread to over 27 states. A well-established population of brown marmorated stink bug was detected in Hamilton, Ontario in August 2012. Field surveys have not yet identified BMSB in Ontario crops, but growers should be on the look-out for the pest.
BMSB has a very broad host range (over 300 plants are mentioned in the literature), including numerous specialty crops, field crops, and wild hosts that can support tremendous populations. These include stone fruit, pome fruit, tree nuts, grapes, berry crops, peppers, tomatoes, sweet and field corn, soybeans, and ornamental trees and shrubs. Other stink bugs have wide host ranges as adults. What makes BMSB different is that is uses so many plants as reproductive hosts - which means injury is caused by both adults and nymphs. The adults are highly mobile and are capable of moving between crops throughout the growing season. The cumulative damage and the extended activity within a crop, coupled with the pests' mobility, translate to additional monitoring and more intensive management.
Stink bugs are shield-shaped insects with piercing-sucking mouthparts. The BMSB has several features that can help you distinguish it from other common stink bugs: two white bands on dark antennae, smooth edge along pronotum or “shoulders” (behind the head) and white triangles alternating with dark areas at the edge of the abdomen. Older nymphs (immatures) have the same banding patterns on the antennae as the adults.
Please visit OMAFRA’s website at www.ontario.ca/stinkbug for updates on the BMSB.
F. Scale Insects
Scale insects are usually tiny and immobile for most of their lives. Only adult males and newly hatched nymphs are mobile. Adult females are wingless, often lack discernable legs, and usually are covered by hard or waxy coating, giving them a distinctive appearance that can sometimes make them difficult to recognize as insects. This waxy covering makes scales very hard to manage; often the only vulnerable stage is the newly hatched nymph which has not yet secreted a shell. Scale insects are related to aphids, plant bugs and leafhoppers and also have sucking mouthparts. They can be found on the bark of woody plants or on leaves, stems or fruit. On soft tissue, their feeding causes yellow spots or lesions while on woody plants it may cause the bark to split. They often blend in with the host plant, making them hard to see. Scales can be divided into three groups:
Armoured Scales - Armoured scales vary from 2-3 mm in length or diameter and posess a hard, protective covering over their bodies that can be removed to reveal the insect underneath. They may be circular, oblong or pear shaped and commonly attack trees and shrubs. When they are present in large numbers, infested bark may appear crusty.
Soft Scales - Soft scales are either bare or enclosed in waxy or cottony secretions. They typically appear more raised than armoured scales when viewed from the side, and are round or oval. Unlike armoured scales, the waxy coating of soft scales cannot be separated from their bodies.
Mealybugs - Mealybugs are soft-bodied insects usually covered with a powdery or cottony wax. The edges of their bodies usually have waxy filaments extending from them. Mealybugs produce honeydew. Unlike other scale insects, mealybugs are mobile throughout their life.
G. Thrips
Thrips are tiny, very slender-bodied insects with distinctively fringed wings. They attack the leaves and flowers of several woody and herbaceous plants. Thrips have rasping mouthparts that they use to scrape off the surface of plant tissue and suck out the fluids. Thrips feed in protected places, often inside a leaf or flower bud, and damage often goes unnoticed until infested buds open to reveal mottled and distorted growth.
BORERS
Borers are beetle and moth larvae that tunnel into the buds, shoots, bark or wood of plants. Infested plants are often damaged beyond repair before you are aware of the presence of borers, so prevention is critical. Signs of damage include canopy dieback, dead branches or areas of bark, bark splitting, shoots/suckers emerging from the trunk, sap flows and sawdust like material on the bark or on the ground.
European Corn Borer - The European corn borer Ostrinia nubilalis (Hubner), a major pest of sweet corn, can also damage peppers, snap beans, potatoes, tomatoes, apples and other horticultural crops. It is an introduced species which arrived in the Great Lakes area in the early 1900's. It is now found throughout eastern and central North America, including most parts of Ontario.
ECB larvae are greasy white to pale grey with two small spots per abdominal segment and a dark brown head. Newly hatched larvae are approximately 3 mm (1/8 in.) long. Fifth instar larvae reach 2.5 cm (1 in.) in length. The abdominal spots are not always present, especially on the younger instars. European corn borer (ECB) egg masses are creamy white and flattened. They are laid in clusters of approximately 30. Each egg is slightly overlapped, giving the egg mass the appearance of fish scales.
In corn, larvae feed briefly on the leaves but soon tunnel into the protection of the stalk. As the crop matures, borers move from the stalk into the ear. In fruiting crops, larvae enter the fruit under the calyx (stem cap). A yellowish-brown sawdust-like residue of droppings may be seen around the hole. Larvae may also enter through the side of the fruit. Fungi or bacteria may invade the entry hole, causing rot.
SOIL PESTS
Pests below the soil line can seriously damage plants before most growers are aware of their presence. It is often difficult to determine the extent of the pest problem or the effectiveness of treatment.
A. Grubs
White grubs are the immature stage of scarab beetles, notably the Japanese beetle, the June beetle and chafers. Grubs of all species have soft, white, C-shaped bodies with tan or brown heads and six prominent, spiny legs. They are quite small when first hatched (3 to 4 mm long), but at maturity reach a length ranging from 2 cm (1 inch) for a Japanese beetle larvae to 4 cm for the June beetle grub. A healthy grub is milky white in colour, with the dark contents of its gut showing prominently through the cuticle at the hind end of the abdomen.
A distinguishing feature among these three species is the pattern of spines occurring on the underside of the tip of the abdomen. On either side of the midline, at the tip, a line of stout spines occurs. In the June beetle grub these two lines are parallel, converging at both ends. In the European chafer grub, the lines of spines are parallel until the tip where they diverge, while in the Japanese beetle larvae the spines form a V-shaped pattern.
June beetle and European chafer adults are very similar in appearance. The European chafer is tan, while the June beetle is dark brown and larger. A distinguishing feature is the presence of a distinct tooth on each claw of the foot of the June beetle, while this tooth is considerably rounded or nearly absent in the European chafer. The Japanese beetle is striking in appearance with a metallic green head and thorax, bronze wing covers and six distinctive white tufts of hair along each side of its abdomen.
Although white grubs are traditionally considered pests of turf, some introduced species have been expanding their host range. In the last several years, white grubs have been found feeding on the roots and crowns of woody nursery plants, and vegetables and fruits in southwestern Ontario. Grubs feed on the roots of emerging seedlings, causing them to wilt and die. When pulled, affected plants will be root-less. In many crops, symptoms of white grub injury on plants include stunted growth and plant dieback. In root crops, white grubs chew deep cavities in roots and tubers, rendering them unmarketable. Some grubs, particularly the Japanese beetle, can also be very destructive as adults, however instead of roots they attack the fruit and foliage of many plants.
B. Root-Feeding Maggots
Maggots are the immature stages of flies. They are typically small, white and legless. Some species attack underground parts, such as the roots of cabbage, broccoli and related plants (e.g. the cabbage maggot).
Cabbage Maggot – Brassica crop seedlings are very susceptible to damage by the cabbage maggot. Young plants from the seedling stage to about a month after transplanting are most vulnerable. They are a considerable threat to crucifers where the root is the marketed portion of the plant (e.g. rutabagas and turnips). They may pose a special threat to some leafy crucifers late in the season when they infest the heart leaves and maggots develop in the leafy portion of the plant.
The cabbage maggot adult is a grey fly, approximately half the size of the common housefly. The small (1 mm, 1/25 in.) white eggs look like tiny grains of rice. The larvae are small (7 mm or 1/4 in.), legless and white. There are three generations of the adult cabbage maggot which lay their eggs in soil near plants in May, early July and mid-August. The first generation causes the most damage. White maggots feed on roots, creating entry sites for rot. Plants become stunted and turn blue-grey; edges of leaves curl. Roots may break off when pulled up. Young plants are usually killed. Older plants survive, but growth is reduced.
Onion Maggot – The onion maggot can be the most damaging insect pest of onions throughout the northern regions where onions are grown. If not controlled, onion maggot can prevent the production of a marketable crop. In Ontario, the onion maggot has become difficult to control. Resistance has developed to many of the insecticides used as granular furrow treatments at seeding.
Onion maggots can cause severe damage every year in some areas but not in others. In May, the first-generation flies lay their eggs near onion plants, seldom near leeks. The white maggots attack the lower stem of the plant soon afterwards and tunnel inside, killing many plants. Yellowing of leaves is usually the first sign of a problem, and indicates that maggots are feeding on the roots. Maggots from second generation flies cause less damage during late July and early August and plants are seldom killed. The last generation eggs hatch in mid-September.
C. Root weevils
Root weevils are the immature stages of snout beetles, named for the mouthparts of the adult stage, which is extended into a distinctive snout. They are white with a brown head and can be distinguished from white grubs by the absence of legs and the uneven thickness of their bodies (they are fatter in the centre than at their ends). The larvae feed extensively on roots of a variety of ornamental and fruit plants, sometimes killing them when numbers are high enough. Adult weevils will feed on fruit and foliage, however adult damage does not usually affect the health of the plant.
D. Wireworms
Wireworms are the immature stages of click beetles. The larvae will attack the roots of a variety of vegetable crops including Carrots, cucurbits, rutabagas, onions, sweet corn, potatoes, sugarbeets, beans, sweet potatoes and peas. They are often confused with millipedes.
The wireworm is unlike most larvae because it is hard-bodied and darker coloured than most maggots or grubs. Wireworms are cylindrical and vary in colour from tan to copper. They range in size from 1-4 cm when mature. The adults are elongated, hard-shelled, dark-coloured beetles which usually have a tapered body and range from 1-3 cm long. Wireworms are most noticeable during the fall and early spring when they come to the soil surface to feed on roots and decaying residue. During the heat of the summer and the winter months, wireworm larvae will migrate deep into the soil for protection.
The life cycle of wireworms ranges from 2-6 yrs depending upon species and location. The larvae are the damaging stage, feeding on roots and seeds in the soil for up to 6 years. Larger seeds may be hollowed out and plants attacked during emergence or soon after transplanting are often killed. Damage to root crops such as carrot and rutabaga closer to harvest can reduce the marketability of these crops and may also predispose them to secondary bacterial infections.
E. Millipedes
Millipedes are often confused with other arthropods commonly found in soil, such as wireworms and centipedes. Millipedes have elongated, cylindrical bodies that range in length from 1-10 cm at maturity. They range in colour from white to grey-black and tend to coil up into a tight spiral when disturbed. They have numerous, uniform body segments. Their most distinguishing feature is their many legs. They have two pairs per body segment. They are usually found hiding or coiled up like a spring.
Millipedes feed on manure and decaying organic matter and generally do little or no harm to plants. However, under certain conditions they will attack seeds or seedlings of a number of Ontario crops, including no/low-till field corn and ginseng, or may tunnel along the surface of roots crops like carrots, sweet potatoes and radish. Millipedes are favoured by moist soils with high levels of organic matter. Numbers may be higher under thick layers of mulch. The millipede is often mistaken for a wireworm, which is orange with six pairs of legs and can cause severe damage to plants. They usually do not need to be controlled.
