Mullein Bug

Table of Contents

1. Introduction
2. Description
3. Biology
4. Damage
5. Monitoring and Management

Introduction

The mullein bug, Campylomma verbasci, can be a devastating pest of Red Delicious and, occasionally, other cultivar such as Northern Spy, Empire and Spartan. However, it is also an important predator feeding on European red mite and green apple aphids throughout the summer months.

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Description

Figure 1. Adult mullein bug.

The mullein bug belongs to the Miridae family, and like the tarnished plant bug, has piercing-sucking moth-parts with which it extracts plant sap or pierces prey. The eggs, inserted into tender woody tissue, are elongate and whitish. The nymphs are tiny, oval-shaped and translucent in colour, becoming pale green as they develop through five instars (Figure 2 and Figure 3).

Figure 2. Young mullein bug nymph feeding on leaf vein.

Figure 3. Older mullein bug nymph.

There are several similar insects that may be confused with mullein bug nymphs. Major features to differentiate these species are illustrated in Figure 4. The adult insect is about 2.5-3 mm. long and grey-brown in colour. The antennae are segmented and the hind legs are spine covered (Figure 3).

Figure 4. Distinguishing nymphal stages of apple pests.

Both nymphs and adults are fast moving, and adults are often quick to take flight is disturbed. There are 2 to 3 generations per year.

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Biology

The mullein bug has two major plant hosts, the mullein plant and apple. Occasionally it will also attack pear, grape, wild rose, serviceberry and oak. The insect overwinters as eggs inserted deep into the bark of one- or two-year-old wood of apple. Nymphs begin hatching during bloom and continue to hatch into the petal fall period. Most years the hatch is very synchronized with peak emergence at early petal fall. However, a cold snap during this time may result in split hatch, making chemical control more difficult.

Nymphs will initially feed on plant sap attained from leaf veins, but will also sting developing fruitlets. Several weeks after petal fall the nymphs become predaceous and begin feeding on small prey such as European red mite and aphids. Nymphs with red bellies are an indication that they have been feeding on mites. Nymphs progress through five instars before becoming adults.

The adult insect migrates 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 apply orchards does indicate that some mullein bugs will remain in the orchard during the summer. During that time they are considered important, but often overlooked, predators of aphids and mites. In late fall, female mullein bugs return to apple trees and lay overwintering eggs into young wood.

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Damage

During bloom to petal fall period, mullein bug nymphs may 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 other cultivars, or if they are immune to mullein bug stings.

Feeding on fruit results in small upraised bumps on the fruit surface. Fruit often receive multiple stings and the large majority of these abort just prior to or during June drop (Figure 5). Affected fruit that do remain on the tree develop small corky warts or bumps surrounded by conical depressions (Figure 6). As the fruit sizes through the summer, distortion of the fruit often occurs.

Figure 5. Mullein bug damage to young fruitlets.

Figure 6. Damage to Red Delicious fruit. 

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Monitoring and Management

Begin monitoring for hatching nymphs during bloom and continue until two or three weeks after petal fall. Since hatch is quite synchronous, frequent monitoring (two to three times per week) is recommended.

Mullein bug are monitored using tapping trays (Figure 7). For instructions on how to construct a tapping trap see the Pest Monitoring section in Chapter Two of Publication 310: Integrated Pest Management for Ontario Apple Orchards . Sample at least 25 trees per block and one limb per tree. Tap each limb two to three times to dislodge the insects and sue a hand lens to positively identify nymphs.

Figure 7. Monitoring mullein bug with tapping board.

Choose limbs with fruit clusters since mullein bug are often present on these. Ideally, tapping should be conducted 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:

• Red Delicious, Spartan - 7 nymphs/25 traps
• Other susceptible cultivars - 10 nymphs/25 traps

In British Columbia, a different monitoring technique was developed several years ago. Sex pheromone traps are placed in susceptible blocks in the fall. Numbers of male mullein bugs caught in the traps were found to roughly correlated to number of females present (1:1 ratio). This allows an early indication of whether a block must be treated the following spring. However, in Ontario this same sex ratio was not found, and therefore this technique is not recommended in this province.

Petal fall insecticides are effective in controlling mullein bug populations that have reached the economic threshold based on limb tapping. Since most eggs have hatched by the time petal fall occurs, it is important to remove honeybee colonies immediately after pollination is complete and apply an appropriate insecticide (See Pest Management Decision Guidelines in Chapter Five of Publication 310: Integrated Pest Management for Ontario Apple Orchards. Note that pre-bloom insecticides, particularly pyrethroids, cannot be counted on to control mullein bug populations into the calyx period, especially during springs with prolonged bloom.

Monitoring of beneficial insects and mites during the summer months should include mullein bug counts. Numbers of mullein bug nymphs and adults should be recorded when conducting green apple aphid terminal infestation counts and in predator limb tap sampling throughout the summer, to give an estimate of the importance of mullein bug as a predatory insect in the orchard, especially in non-susceptible cultivars.

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