The Online Gardener's Handbook 2010
Chapter 1: Causes of Plant Injury
Insects and Other Invertebrates Causing Plant Injury

Table of Contents

  1. Introduction
  2. Defoliators and Chewing Invertebrates
  3. Sucking Insects
  4. Borers
  5. Gall Markers
  6. Soil Pests


A wide variety of insects, spiders, snails, slugs and other invertebrates are commonly found in and around home gardens. While some are damaging to garden plants, most 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 in your garden is not necessarily a reason to undertake control measures. Ensure it is actually damaging your garden before trying to remove it, keeping in mind that many landscape plants can tolerate considerable cosmetic damage without significant long term impacts. Additionally, remember that any control measure used against a pest insect could also be harmful to their natural enemies or to pollinators.

Many kinds of insects and mites cause injury to ornamental plants and it is not possible to discuss them all in this publication. Only commonly observed problems are mentioned in this book. Invertebrate pests can be broadly divided into five categories according to their feeding habits/mouthparts and the damage they cause - chewing insects that defoliate leaves or attack fruit, sucking insects, wood borers, gall makers and soil pests.

Some damage is distinctive, allowing identification of the insect that caused it without even seeing the pest at work (see Table X Location of Plant Damage Symptoms and possible Causes). 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.

Fruit- and Leaf-Feeding 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. 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 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.

Fruit- and Leaf-Feeding 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.


Leafminers are the immature stage of some small insects, typically flies (maggots) or caterpillars, which develop within leaf tissues. 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.

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).


Sawflies are related to bees, wasps and ants. The adults are stout-bodied, have two pairs of wings and resemble small, amber-coloured bees. The larvae of most species look similar to caterpillars, with 3 pairs of true legs on the thorax, close to the head. They usually have seven or eight pairs of prolegs (with no hooks), located on their abdomen, a feature which distinguishes them from caterpillars. Many sawflies feed in colonies and quickly strip leaves from their host plants. Others, including some infesting apple and pear, are found singly.

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.


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 projectionsnear 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.


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 posess 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 toxin 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.


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 gardens 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.

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.

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 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.


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 are beetle and moth larvae that tunnel into the buds, shoots, bark or wood of trees and shrubs. Some species attack healthy trees, but most attack trees and shrubs already under stress from another cause.

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. Emerald ash borer and Asian long-horned beetle are examples of invasive alien borers that have received a lot of media attention due to their destructiveness.

Gall Makers

Galls are abnormal growths produced by a plant in response to insect irritations. They can occur on any part of a plant. Causes include feeding, stinging, egg-laying or toxin injection. Galls generally benefit the pest by providing a protective shelter for the insect to complete its life cycle. Aphids (and aphid-like insects such as phylloxera), mites, midges (flies), gall wasps and some beetle and moth larvae can all produce galls. Galls very greatly in size, colour and complexity, but each insect species produces a characteristic gall formation. They rarely kill plants, but can greatly affect their appearance.

Soil Pests

Pests below the soil line can seriously damage plants before most gardeners are aware of their presence. It is often difficult to determine the extent of the pest problem or the effectiveness of treatment.


White grubs are the immature stage of scarab beetles, notably the Japanese beetle, the June beetle and chafers. They have plump, milky-white bodies; brown heads; three pairs of legs and move into a distinctive "C" shape when disturbed. Larvae of white grubs are very similar in appearance and cannot easily be distinguished from one another.

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. Some grubs, particularly the Japanese beetle, can also be very destructive as adults, feeding however instead of roots they attack the fruit and foliage of many plants.

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).

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.


Wireworms are the immature stages of click beetles. They are orange, hard-bodied larvae with six legs. The larvae will attack the roots of a variety of vegetable crops including carrots, potatoes, onions and corn. They are often confused with millipedes.

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