Excerpt from Publication 310, Integrated Pest Management
Table of Contents
The mullein bug, Campylomma verbasci (Meyer), can be a devastating pest of Red Delicious, and occasionally other cultivars such as Northern Spy, Empire and Spartan. Mullein bug is also an important predator, feeding on European red mite and green apple aphids throughout the summer months.
Eggs are 0.8 mm long, white and flask shaped. Mullein bugs develop through five nymphal instars to adults. Nymphs are 0.5-2.5 mm long, yellow, with a pointed head and red eyes (Figure 4-72). There are several similar insects such as aphids and leafhoppers that can be confused with mullein bug nymphs. Major features to differentiate these species are illustrated in Figure 4-73. Adults mullein bugs are 3 mm long, oval and light green to tan in colour (Figure 4-74). Antennae are segmented and hind legs have black spots and are spine covered.
Figure 4-72. Young mullein bug nymph
White apple leafhopper
Figure 4-73. Distinguishing nymphal stages of apple pests
Figure 4-74. Adult mullein bug
The mullein bug has two major plant hosts, the mullein plant and apple. Occasionally it also attacks pear, grapes, wild rose, serviceberry and oak. The insect overwinters as eggs inserted deep into the bark of one- or two-year-old wood of apple. Eggs begin hatching during bloom and continue to hatch into the petal fall period. Most years the hatch is synchronized with peak emergence at early petal fall, but a cold snap during this time may result in split hatch, making chemical control more difficult.
Nymphs initially feed on plant sap attained from leaf veins, and also sting developing fruitlets. Several weeks after petal fall, nymphs become predaceous and begin feeding on prey such as European red mite and aphids. Nymphs with red bellies are an indication the nymphs have been feeding on mites. Nymphs progress through five instars before becoming adults. Both nymphs and adults are fast moving, and adults are often quick to take flight if disturbed. The adults migrate to mullein plants, common along ditch banks and the sides of roadways in Ontario, where they feed throughout the summer months. Season-long monitoring in apple orchards indicates some mullein bugs remain in the orchard during the summer. During that time they are considered important predators of aphids and mites. In late fall, female mullein bugs return to apple trees and lay overwintering eggs into young wood. There are two to three generations per year.
During bloom to petal fall period, mullein bug nymphs cause economic losses to certain varieties of apple, particularly Red Delicious and Spartan. Northern Spy, Empire, Cortland, Gala, Jonagold and Golden Delicious are also sometimes affected. McIntosh and other cultivars seem to be largely unaffected, but it is not known whether mullein bug does not generally attack fruit of these cultivars, or if these cultivars are immune to mullein bug stings. Feeding on fruit causes small upraised, reddish bumps on the fruit surface (Figure 4-75). Fruit often receive multiple stings and the large majority of these abort just prior to or during June drop. Affected fruit that remain on the tree develop small corky warts or bumps surrounded by conical depressions (Figure 4-76). As affected fruit sizes through the summer, it becomes distorted. If uncontrolled, mullein bug can damage up to 75% of apples in Ontario orchards.
Figure 4-75. Mullein bug damage to young fruitlets
Figure 4-76. Damage to Red Delicious fruit
Begin monitoring for nymphs during bloom and continue until two or three weeks after petal fall. Hatch is usually quite synchronous and frequent monitoring (two to three times per week) is recommended. First emergence and threshold can be missed if only monitoring once a week, and may result in fruit damage. Monitor mullein bug using tapping trays (Figure 4-77). For instructions on how to construct a tapping trap, see the Appendix E, How to build a tapping tray on page XX. Sample at least 25 trees per block and one limb per tree.Tap each limb two to three times to dislodge the insects while holding the tray below the branch. Check trap after tapping each branch. Newly hatched nymphs are quite difficult to see, and a hand lens is recommended to positively identify nymphs.
Figure 4-77. Monitoring mullein bug with tapping board
Choose limbs with fruit clusters since mullein bug are often present on these. Concentrate on sampling blocks of susceptible cultivars and blocks where mullein bug has been a problem. Conduct tapping on sunny days, once temperatures have warmed. Avoid tapping on cold, overcast days or when it is raining.
The following economic (action) thresholds can be used:
Once mullein bug populations have reached the economic threshold (usually around petal fall) insecticides should be applied. Since some of the products targeting mullein bug can have negative impacts on bees it is important to remove honeybee colonies immediately after pollination is complete before applying an insecticide. Do not count on pre-bloom insecticides - particularly pyrethroids - to control mullein bug populations into the calyx period, especially during springs with prolonged bloom. Economic thresholds for mullein bug are based on pest density, but it is important to note that damage caused by this pest also is affected by availability of food such as pollen, nectar, plant nitrogen or animal prey. As a result, high populations of mullein bug may not cause significant fruit damage.
During the summer mullein bugs are important orchard predators,
so include mullein bug counts in monitoring of beneficial insects
and mites during the summer months. Record numbers of mullein bug
nymphs and adults when during terminal inspections and in limb tap
sampling throughout the summer.
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