Blueberry Insect Pests Management Review

Introduction. According to the data from the 1997 Census of Agriculture, Massachusetts raises 469 acres of highbush (tame) and 278 acres of lowbush (wild) blueberries. When combined, blueberries represent about 57% of the berry production in the Commonwealth (excluding cranberries). The official estimated cash value of this crop is $1,575,000 highbush and $240,000 lowbush. The actual farm-gate value of this crop is likely to be somewhat more, perhaps a lot higher. Effective management of insects diseases and weeds, as well as good soil management, plant nutrition and pruning, are key to setting the stage for profitable blueberry production. Of course, harvesting and marketing factors come into play, too. But, first we have to get a good marketable crop on the bush.
Over the last 2 years, the UMass Extension Small Fruit Program has conducted some intensive work on blueberry production, focusing on field testing of IPM practices derived from work in other states, notably New Jersey, New York, and Maine. This paper will review the aspects of that work that had to do with the management of primary insect pests of both highbush and lowbush blueberries in Massachusetts.

It is important to keep in mind the larger perspective of IPM, even while looking at specific elements of the pest complex found in blueberries. Why? Because what we do to manage one aspect of the crop, often has an effect on another aspect of the crop. The important elements of IPM include:

  • Insect pest management (scouting)
  • Disease management (weather monitoring)
  • Weed management (annual survey)
  • Nutrition management (soil test, tissue test)
  • Growth Management (pruning, mulching)
  • Sprayer calibration

I mention this to encourage the reader to remember the holistic nature of the system and that all elements working together are needed for effective and profitable management of any crop.

Managing Insects in Blueberries. The insect pest complex found in blueberries consists generally of blueberry maggot fly, cranberry fruitworm, cherry fruitworm, aphids and leafhoppers (virus vectors), various leafrollers, blueberry blossom weevil, blueberry tip borer, blueberry stem borer, dogwood borer, plum curculio, scale insects, Japanese beetles, yellow jackets, and sometimes gypsy moth. This paper will discuss the identification, life cycles, and management of the two most significant of these insects for New England production, cranberry fruitworm and blueberry maggot.

Cranberry Fruitworm (Acrobasis vaccinii)

Adult cranberry fruitworm (CFW) moths are active in blueberries from June through early August. The front wings are gray-brown with two very small dark dots near the front margin. There are also two white markings, one toward the base of the front wings and the other one around the middle of the front margin. The hindwings are also gray-brown. They are seldom seen because they are active only at night. During the day they stay hidden among the crop plants or weeds.

Eggs are laid singly in the calyx end, or blossom end, of berries by the adult female moths while the berries are small and green. Rarely does another moth deposit an egg on a berry that already has an egg present. As they near hatch, the eggs develop an irregular orange streak. This is generally only 3-5 days after being laid. When looking for eggs it is best to use a magnifying lens of some kind as it is nearly impossible to see the eggs.

The CFW larva (caterpillar) is mainly green with some brownish-red coloration on its top surface and measures about 1/2 inch or more at maturity. Once larvae have emerged from the eggs, they immediately crawl to the stem end of the berry. They then burrow their way into the berry, close the entrance hole with silk, and begin to feed. They are found first within green fruit which will turn blue prematurely. This characteristic offers growers a “red (or blue!) flag” alerting them to the infestation. They feed from early June through harvest, and during that time, each larva will typically move from one berry to another, hollowing out several berries in a cluster. Each larva will molt roughly five times (going through six instar stages). Feeding reduces the crop and spoils marketability of the berries. Larvae will consume from 3-6 berries, filling them with brown frass, and web together fruit with silk.

When CFW larva reaches its last instar stage, it will drop to the ground and spin a cocoon of silk, leaf litter and soil, thus beginning its hibernaculum stage. This is the dormant stage in which the larva lives out the winter months. In the spring the hibernacula will begin to pupate and within about five weeks the adult form, or moth, will emerge. These moths emerge over a long period between the end of May and the end of July. Within a few days after emergence the females will mate and begin laying eggs in the calyx end of the fruit.

Cranberry Fruitworm Management. If damage has been noted in a planting, scouting for the pest using pheromone traps the following year is recommended. These are wing traps that contain a lure which is impregnated with a synthetic replication of the female CFW sex pheromone. As such, the traps attract male moths which are caught by the sticky surface of the floor of the trap. Traps should be checked at least once each week, the number of CFW moths recorded, and then moths removed in order to determine the next week’s catch. Lures are effective for 2-3 weeks and should be replaced periodically during the scouting period which lasts for 5-8 weeks beginning mid-May.

Recommendations from New Jersey and New York state that insecticide applications should be timed for one week after a peak in trap catches is observed. A second spray 7-10 days later is also recommended. In severe situations, it may be necessary to apply a third spray. Traps should still be monitored after the spray program commences to determine the effectiveness of the sprays and determine if additional sprays are needed.

Control measures can also be reliably timed to the plant’s stage of growth. In fact, the “old” recommendation for control was an insecticide spray when the green berries begin to touch each other. In our observations this is relatively close (about 3-5 days before) we observed peaks in our trap catches, making the applications 7-10 days earlier than our trap-based recommendations. Either way, a second follow-up spray is recommended. The advantage of using a trap-based system is in determining if CFW is, in fact, present in a given planting.

Another way to determine if a second or third spray application is needed is by examining berries for eggs. A hand lens will be needed for this purpose. If 1 or more eggs are found per 100 berries, a spray 10-11 days after the previous application in recommended. Proper management should limit losses to less than 3 percent of the fruit.

In small plantings, CFW can be effectively controlled by picking off infested berries that are easily detected by their early color. Both egg and larval parasites have been associated with the cranberry fruitworm. Phanerotoma franklini is the most common of these species, but it has never been found in densities high enough to provide effective control.

Cherry Fruitworm (Grapholita packardi)

The cherry fruitworm adult stage is a small, dark gray moth with chocolate colored markings on the wings. These moths have a wingspan of 8-11 mm (3/8 to 1/2 inch). As with cranberry fruitworm, they are seldom seen because they are active at night and hide under leaves during the day. The cherry fruitworm larva (caterpillar) at maturity is orange-red and about 1/4-1/2″ long. It is found within developing and ripening berries. Feeding reduces the crop and spoils marketability of the berries. Hatching larvae bore into the calyx cup (blossom end) of the berry, feed until about half-grown, and then move to a second fruit. (This is distinct from the CFW described above.) The two infested berries are usually joined by silk.

Cherry Fruitworm Management. When damage is severe, treat in the following year with insecticide. Monitor cherry fruitworm utilizing the same traps as are used for Cranberry Fruitworm.

Blueberry Maggot (Rhagoletis mendax)

The blueberry maggot (BBM), or blueberry fruit fly, is the most important and pervasive insect pest of blueberries in New England. The female fly is about 3/16″ long with a wing span of approximately 1/3″. The abdomen is pointed and black with 4 white cross-bands. The thorax is basically black with a small white dorsal spot. The eyes are reddish. The male fly is somewhat smaller than the female and has a rounded abdomen with only 3 white cross-bands. The wings of both sexes are clear, marked with characteristic M- or W-shaped black bands (depending on your aspect when viewing them).

The first flies begin to emerge from the soil in late June or early July and continue to emerge until early August. After emerging, the flies, which live for about 30 days, spent 1 to 2 weeks resting and feeding on dew, insect honeydew, and secretions on foliage. During this resting and nourishment period, the adult females become sexually mature and mate. Once mated, the females seek ripening blueberries in which to lay eggs. Each female fly may lay up to 100 eggs in a period of 15 – 25 days.

The female fly punctures the skin of the blueberry in order to deposit a single white elongate egg. The fly then leaves behind an oviposition-deterring pheromone on the surface of the “occupied” berry. The chemical deters other flies from laying an egg in the same berry.

In 7–10 days the egg hatches and the larva (maggot) begins feeding. The first maggots appear in the berries about mid-July. The full-grown larva is whitish in color. It is about 5/16 “long, round, pointed at one end, and blunt at the other. As the larva feeds and grows, the berry begins to shrivel. After two or three weeks of feeding, the larva becomes full-grown and the berry is almost completely destroyed. Infestations tend to increase rapidly to a peak by mid-August. At this time, many of the larvae begin exiting the berries and dropping onto the soil. However, undetected infested berries that are harvested contaminate pack-out, causing problems with customers.

Fallen larvae burrow into the soil to 1”- 2” to pupate encased in a tough, brown outer skin called a puparium from which the adult emerges in early summer. In the first year after pupation approximately 85 % of the adult insects emerge. In the second year roughly 10 % emerge. In the third and fourth years the remaining 5 % of the flies emerge. This ensures the survival of a portion of the population in the event of high mortality in any given year. This is important to keep in mind when trying to manage this insect. Some of the pupae will remain in the soil to emerge the following year.

Note: There are several other species of fruit flies that look similar to the blueberry maggot including the apple maggot, black cherry fruit fly, the cherry fruit fly, and the walnut husk fly. These flies are distinguished by their wing patterns. The blueberry maggot has a continuous dark wing pattern. The black cherry fruit fly has a small, oval, clear spot in the wing and the cherry fruit fly’s dark wing pattern is not continuous. The walnut husk fly’s wing pattern closely resembles that of the blueberry maggot. However, on close inspection, it can be seen that the walnut husk fly’s wing pattern is not continuous. The walnut husk fly is not very common in New England, since its host, the black walnut, is uncommon.

Blueberry Maggot Management. The highest BBM fly populations are generally associated with fields where soil moisture remains high enough throughout the summer (due to irrigation, high organic matter, or tall bushes that shade the ground) to prevent desiccation of the pupae. Effective management is based on accurately identifying the timing of emergence and the density of the pest in a planting. Traps are useful for monitoring both the emergence and density of this insect pest. Red sticky spheres or yellow sticky rectangle traps, originally developed for use in trapping apple maggot fly (Rhagoletis pommonella), a close relative, can be used to monitor BBM populations in the planting. In addition, a green sphere trap was introduced. It is thought that the flies perceive all of these traps as foliage, and, thus, a feeding area. Baiting of the traps increases their effectiveness. Once the flies are attracted to the traps, they are caught in the sticky material and can be counted. Of the options available, the sticky yellow traps with protein hydrolysate or ammonium acetate bait may be preferred for their ease of use, cost, and effectiveness.

For highbush blueberries, sticky traps should be hung in upper half of the canopy and about half-way from the center of the bush to the outer edge of the foliage. All fruit and foliage within 8 inches from the trap should be cleared away, and all traps positioned so that there is as much foliage and fruit surrounding them at this distance as possible. Trap density should be 2-4 traps per acre. In small plantings, it may be possible to trap this insect out with sufficient trap density. If bushes are young and small, traps can be suspended from stakes.

Traps should be places in the field prior to expected emergence of the flies, that is, before any fruit ripening begins to occur. Traps should be checked once a week and an action threshold of 1 BBM fly per week is common and has been shown to be effective. When checking the traps, any BBM flies found should be counted and removed from the trap. This way, only new catches are counted the following week. Choices of materials and rates change over time and growers should consult current recommendations for this information.

Baits on the traps are effective for about 2 weeks. Either replace the whole trap every 2-3 weeks or use bait replacement “super-chargers” added to traps every 2-3 weeks. Traps remain in the field from mid-June through harvest and should be checked weekly the entire time.

In lowbush blueberries, the traps should be suspended from stakes with the underside 6 to 10 inches above the tips of fruiting blueberry plants. To be most effective the yellow, rectangular traps should be placed in a “V” position, with the yellow sticky surface directed downward. The traps should be placed about 25 feet from the perimeter of the blueberry field, especially facing unmanaged blueberry areas. The flies prefer protected areas; therefore, traps should be located 1 to 2 feet from bushes or thick patches of weeds. Weedy areas, with sweet fern or barrenberry are good locations for trap placement. Use one trap per acre in fields under 10 acres. In larger fields, a trap should be placed every few hundred feet along the border. This may figure out to be one trap every two or three acres, depending on the size of the field. The traps should be replaced every 2 to 3 weeks. Therefore, three traps per acre will be needed at each trap site to monitor the field throughout the season.

As with the highbush system, BBM traps should be placed in the fields 7 to 10 days before anticipated fly emergence. The monitoring system is slightly different from highbush, however. Once the first fly is captured, the traps should be checked every 3 to 4 days. After checking a trap, record their numbers and then remove all blueberry fruit flies. If six or more blueberry fruit flies are found on any one trap in a single visit or if a cumulative total of ten flies are captured on a single trap in more than one visit then some control measure should be considered. This requires monitoring each individual trap for cumulative totals.
Monitoring blueberry fruit flies in individual fields can reveal the approximate time of their emergence. Monitoring information will also show the relative abundance of flies throughout the season. This information can help the grower avoid using insecticides unnecessarily and to properly time any insecticide applications which are used.

Conclusion. Blueberries are affected by numerous insect pests, but cranberry fruitworm and blueberry maggot fly represent the most significant direct threat to marketable yield in New England. Effective management of these pests will contribute greatly to the profitability of both highbush and lowbush blueberries.

By: Sonia Schloemann, Dept. of Plant and Soil Sciences, Univ. of Massachusetts, Amherst, MA 01002

Originally published: Proceedings. 1999. New England Vegetable and Berry Growers Conference and Trade Show, Sturbridge, MA. p.177-182.

Updated by: Mary Concklin, UConn IPM. 2012

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