UMCE Blueberry Maggot Fact Sheet

Monitoring for the Blueberry Maggot
Rhagoletis mendax Curran

Fact Sheet No. 201

Life Cycle

The blueberry maggot, or blueberry fruit fly, is the major insect pest of blueberries in Maine. The first flies (Fig. 1) begin to emerge from the soil in late June or early July. Flies 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 mates. 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 to 25 days.

Blueberry Maggot Adult
Figure 1.

The female fly punctures the skin of the blueberry with a long, pointed structure called the ovipositor, which can be withdrawn into the insect's abdomen. Once the ovipositor is in the berry, a single white elongate egg is deposited. The fly then drags the ovipositor over the surface of the berry leaving behind a chemical called an ovipositing-deterring pheromone. The chemical deters other flies from laying an egg in the same berry for a few days.

In 7 to 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 7.75 mm (5/16 in.) 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 (Fig. 2), and the berry is almost completely destroyed. The infestation, if present, increases rapidly to a peak by approximately the second week of August. At this time, many of the larvae begin exiting the berries and dropping onto the soil.

Blueberry maggot
Figure 2.

The larvae burrow into the soil to a depth of 1 to 2 inches to pupate. The insect spends the winter in the soil in this resting stage, 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 percent of the adult insects emerge. In the second year roughly 10 percent emerge. In the third and fourth years the remaining 5 percent of the flies emerge. This ensures the survival of the species. If a calamity should befall the population in any given year, some of the pupae would still be in the soil to emerge later.

Description (adult)

The female fly is about 4.75 mm (3/16 in.) in length with a wing span of approximately 8 mm (1/3 in.). The abdomen is pointed and black with 4 white cross-bands. The thorax is basically black with a small, backward-pointing, white projection. The two large compound eyes are reddish. The male fly is somewhat smaller than the female and has a rounded abdomen with only 3 white cross-bands (Fig. 3). The wings of both sexes are clear, marked with characteristic black bands (Fig. 5).

Maggot abdomen
Figure 3.

Fly Wings
Figure 4.

There are several other species of fruitflies that look similar to the blueberry maggot including the 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 Maine, since its host, the black walnut, is uncommon.

Alternate Hosts

The blueberry maggot attacks both lowbush blueberries and highbush blueberries. This insect has also been reared from huckleberry (Gaylussacia baccata).

Monitoring Technique

Type of Trap

Yellow rectangles coated with a sticky material are used for monitoring the blueberry fruit fly. These rectangles can be made and painted by oneself or purchased pre-coated and baited. The pre-coated and baited traps are easier to use and are available from commercial suppliers. If you are interested in purchasing the traps, please contact Cooperative Extension for sources.

It is thought that the flies perceive these yellow traps as foliage, and, thus, a feeding area. Also an ammonium-based bait on the commercial traps aids in luring the flies to traps. Once the flies are attracted to the traps, they are caught in the sticky material (Fig. 5).

Figure 5.

Trap Replacement

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 and consistent with the action thresholds indicated below, the yellow, rectangular traps should be placed in a "V" position, with the yellow sticky surface directed downward (Figs. 6 & 7).

Trap placement
Figure 6.

Yellow sticky trap
Figure 7.

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 will be needed at each trap site to monitor the field throughout the season.

Action Thresholds

The traps should be placed at the selected sites 7 to 10 days before anticipated fly emergence. 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.

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.