Insect Note #B-2

Prepared by:
G. A. Pearson, J. R. Meyer, and K. A. Sorensen

Dated 9/90
Placed on the Web 4/95 by the Department of Entomology, NCSU

The Blueberry Maggot

Rhagoletis mendax Curran; Diptera: Tephritidae

The blueberry maggot is the most serious insect pest of cultivated blueberries in North Carolina.

Damage

The blueberry maggot is the immature stage of a fly. It is of major concern because it feeds inside ripening fruit and may remain there for some time after harvest. Infested berries cannot be separated from sound berries during harvest and packing, and maggots may emerge from the berries at the point of sale.

Fruit containing maggots may be seized and condemned by the FDA. Federal inspectors monitor interstate blueberry shipments, and processors and other buyers perform tests to detect contaminated berries. Maggot infested berries become soft, mushy, and leak, causing a sticky, unattractive pack.

The blueberry maggot feeds on all varieties of huckleberries and blueberries. Because of continuous migration from the wild, it is difficult to keep cultivated blueberries free of maggots if they are grown near areas containing stands of wild plants.

Life Cycle

This insect overwinters in a brown, seedlike resting stage or puparium, about 1/8 inch long, buried from 1 to 6 inches deep in the soil. In North Carolina, flies begin emerging from the soil in late May, continuing through the months of June and July.

The adult fly is very active and slightly smaller than a house fly. It can be recognized by a distinctive pattern of black bands running diagonally across each wing, white bars on each side of the thorax, and a white spot at the posterior tip of the thorax. The body of the fly is black. Females begin to lay eggs 7-10 days after they emerge as adults. Females may lay eggs in either ripe or green berries. The eggs are inserted just beneath the skin of the berry and hatch in about 5 days.

Blueberry maggot larvae have tapered, worm-like bodies with no eyes, legs, or antennae. Tiny newly hatched larvae are colorless, and cannot be seen until they are about a week old. Later, growth of the larvae and the liquifying of infested berries makes their detection easier. The maggots grow to about 1/2 inch in length in about 20 days. Then they drop to the ground and burrow into the soil to pupate.

Most adults emerge from their overwintering puparia the following summer, although a small percentage may emerge the second summer.

Control Recommendations

The control of this insect is complicated by its long emergence period, its migratory tendencies, and the fact that it does not start its attack on the fruit until harvest has already begun. These factors prevent the use of residual insecticides, making frequent applications of relatively non-toxic, non-residual insecticides necessary.

IPM and Scouting

Not all blueberry fields have maggot infestations, and infested fields may only contain small "hot spots" of fly activity. Regular monitoring of blueberry maggot emergence should be made with yellow baited stick traps. This serves three purposes: to detect blueberry maggot populations before they reach a damaging level; to optimize timing of insecticide sprays; and to reduce the amount of insecticide by spraying only those areas actually infested.

Sticky traps may be purchased or home-made. The traps are bright yellow and have a protein hydrolisate bait. Both the color and the bait attract newly emerged flies. Place traps knee-high to waist- high near fruit in a shaded part of a bush. Four traps/acre should be set out, or traps may be placed using the method described below. Two adults per trap per week is the recommended chemical treatment threshold.

Some counties have extension scouting programs, or professional IPM scouting services. Your county Agricultural Extension Office can advise you on the needs for control measures.

A new method to predict where maggot infestations will occur has been developed using risk factor analysis. These statistics are similar to those used by the insurance industry to set your rates. Just as some people may have driving habits that put them at a higher risk of an auto accident, some blueberry fields have characteristics that put them at a higher risk of a maggot infestation than others.

By using the new procedure to predict infestations, one avoids placing many sticky traps in each field to detect infestations. Traps are used only in areas predicted to be infested to confirm the presence of adult flies and to optimize spray timing. This should reduce trapping materials and scouting time. This procedure also can be used to identify when new blueberry plantings are likely to become infested.

Fortunately, one does not need statistical computer software to use this model. Finding the probability of an infestation in a given spot is fairly easy with just a yard stick and a calculator. The model relies on two blueberry bush characteristics: the height of the bush and the amount of shade created by the bushes at noon. To calculate the probability of a maggot infestation at a site, click here. Any bushes under 3 1/2 feet in height will have a 0% chance of an infestation and should not be trapped. Values for infestation probabilities will change as the bushes are pruned. It is recommended that measurements be made in early May, prior to fly emergence.

Some sites predicted to be infested will not have any maggots present. These "false positives" are probably sites that are suitable for maggot colonization, but flies have not yet become established. These sites should be trapped each year and watched carefully for maggots.

The probability level at which one decides to place a trap may vary, depending on how conservative one wants to be. For example, choosing 40% as the threshold value for setting a trap reduces the chance of missing a fly infestation, but also increases the number of uninfested sites likely to be trapped and scouted.

Given the current zero tolerance level for maggots, it is probably best to use a conservative 40% level the first year, until one is confident in the use of the prediction method. Keeping records of the actual infested areas and their predicted chances of infestation will allow you to evaluate the level of probability best for a particular area. Using a 50% cutoff value to initiate treatment, this model predicted infestations 100% of the time at test sites chosen in Duplin Co.

This model only predicts the ability of a site to support a blueberry maggot population from year to year. Once a site is found to be infested, it should be trapped for the next 2-3 years, since pupae are known to take more than one year to complete development. If there is immigration of adult flies from adjacent wooded areas, an infestation may occur that will not be predicted by this model.

This model was developed on non-irrigated highbush blueberry fields, and should NOT be used on irrigated fields. Evidence gathered during development of this model suggests blueberry maggot survival is closely linked to soil water balance. Thus, irrigation may allow more puparia to overwinter successfully and increase chances of an infestation. This proposed relationship between maggot survival and soil moisture partially explains the common observation that "black" soils are likely to have maggot infestations.

If you irrigate blueberry fields, be aware that maggot problems may increase. It is recommended that you place about 4 traps per acre to determine the presence of flies and follow the spray program described above.

Your county Cooperative Extension Office can advise you on the use of this model, interpret results, and answer questions.

Chemical Control

To control the blueberry maggot apply a spray that contains 2 lbs. of 25% wettable powder malathion per 100 gallons of water. Malathion ultra low volume at 10 oz. per acre by ground or aerial application is also effective if a thorough coverage is achieved. Spray when adults first appear (usually the last week in May) and every 7 to 10 days during the egg-laying period. DO NOT APPLY MALATHION ON THE DAY OF HARVEST. A highbush blueberry grower may have to use as many as five applications of insecticide to give protection over the long emergence and migratory period of the fly in the field. Consult the most recent North Carolina Agricultural Chemicals Manual for more information on insecticide compounds and rates.

Natural Control

There are few significant parasites of the blueberry maggot. For the most part, biological control agents are ants and other soil predators which eat the fly's resting stages as they overwinter in the soil. Perhaps the best non-chemical method to control the flies is to prune bushes to reduce the shade they cast, and to non augment the soil moisture in late summer and early spring--times when the flies' resting stages are most likely to dry out in hot sands. Untreated, infested fields may serve as a source of infestation for surrounding areas, so be conscientious in your control efforts.

The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsements by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. All pesticide recommendations provided in this publication apply to pest control in North Carolina. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain information about product usage in your locale and examine a current product label before applying any chemical. The information presented on pest biology is based on conditions in North Carolina and may not be applicable to your locale. For further information, we recommend that you contact the Cooperative Extension Service office in your county.