Apple Maggot

Table of Contents

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

Introduction

The apple maggot, Rhagoletis pomonella (Walsh), is a major pest of apple in Ontario. A native insect, it also occasionally attacks plum, cherry, peach, pear, hawthorn and Cotoneaster sp.

Due to import restrictions in some other countries (and British Columbia), a zero tolerance for fruit infestation by apple maggot is observed in most commercial orchard in Ontario.

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Description

The apple maggot belongs to the Tephrididae family of lies. Slightly smaller than a house fly, the adult has a white dot on its thorax and a characteristic black banding on its wings (Figure 1). The F design of this banding is a key diagnostic feature, distinguishing it from similar species that may occasionally appear in orchards (see Figure 3). The female fly is slightly larger than the male, has four white lines on the abdomen and a squared-off posterior end (Figure 1). The smaller male has only three white lines and a rounded abdomen (Figure 2).

Figure 1. Female apple maggot fly - note tapered abdomen.

Figure 2. Male apple maggot fly - note rounded abdomen.

Figure 3. Wing patterns of apple maggot and related species.

The larvae, which causes the actual damage to the fruit, resembles a typical fly larva or maggot. Even in heavily infested fruit, it is often difficult to detect due to its pale, cream colour and small size.

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Biology

There is only one generation of apple maggot per year in Ontario, although occasionally a partial second generation can occur when early maturing cultivars are attacked.

The insect overwinters as a pupa in the soil. Adults emerge from late June through September, with peak flight occurring in August (see Figure 4). Emergence is closely linked to soil moisture levels. In dry years the pupae may actually remain in the soil until the following growing season.

Male flies generally begin emerging before the females, but by peak emergence the sex ratio is about 1:1. Both sexes initially feed on nectar and other sugar sources before becoming sexually mature seven to ten days after emergence.

Figure 4. Typical emergence pattern of apple maggot adults.

After mating, each female will produce as many as 300 eggs, which are laid over a period of three to four weeks. The eggs are laid (oviposited) singly under the skin of developing fruit. More than one egg may be laid into each fruit. The incubation period is three to seven days. Upon hatching the larvae burrow into the fruit where they feed on the pulp. The characteristic "railroading" is a result of the brownish tunnels observed when infested fruit is cut open.

After three to four weeks of feeding, heavily infested apples often fall to the ground. Apples with only one or two stings will often remain on the tree until harvest. The mature larvae leave the fallen fruit and tunnel into the soil to a depth of 2-5 cm . At this point the larva forms a pupa and goes into diapause. The pupae can remain in the soil for up to five year if environmental conditions (moisture and temperature) are not favourable for emergence).

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Damage

The external oviposition "stings", produced when the female lays her eggs on the fruit's surface, are difficult to detect. They resemble a small pinprick, which is slightly darker than the fruit's ground colour and often appear slightly sunken, or dimpled (Figure 5).

Fruit damage is a result of feeding and tunneling of the larvae inside the fruit (Figure 6). This injury has led to the common nickname, "railroad work". As the larva grows and tunnels become larger and begin to discolour. Invasion by the disease fungi, Alternaria spp. and Pseudomonas spp. and Pseudomonas spp., may lead to further decay (Figure 7).

Figure 5. Apple maggot "stings" causing dimpling on fruit surface.

Figure 6. Larval tunneling in fruit.

Figure 7. Bacterial breakdown of flesh due to maggot feeding in fruit.

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Monitoring and Management

Apple maggot are monitored using visual traps consisting of two red spheres and a scented, yellow rectangular board, both coated with a sticky material (Tangle trap) to capture the flies (Figure 8). See Appendix X of Publication 310: Integrated Pest Management for Ontario Apple Orchards for sources of monitoring supplies. Five such "trap sets" are placed in trees along the edge of the orchard, adjacent to wood lots with wild hosts or abandoned orchards. The trap sets are placed at eye level facing out from the orchard. The yellow board is placed between the red spheres with 15 to 20 cm between the yellow board and each red sphere. Foliage within 30-50 cm of the trap set should be removed to make the trap more visible. The distance between trap sets should be 60-100 metres.

The trap sets should be placed in orchard by mid- to late June and monitored twice weekly. Many insects will be attracted to the traps, especially the yellow board. This mean the traps set will need frequent cleaning by scraping off trapped insects and re-coating with Tangletrap.

Figure 8. Apple maggot trap set.

Male flies, which usually emerge first, and female flies are initially attracted to the yellow boards which resemble nectar sources in colour. Once sexually mature, female flies are attracted to red spheres which mimic apple (oviposition sites). This pattern of catches is, however, not always 100% dependable, since female flies are often already sexually mature when they enter the orchard from outside sources.

The first insecticide spray should be applied seven to ten days after the first apple maggot fly is caught on a yellow board or immediately after the first female fly is captured on a red sphere. Subsequent sprays are applied every 14-21 days or as continuing trap catches of apple maggot are continuous and heavy, spray intervals should be shortened to 14 days. Timing of apple maggot sprays should correlate with those for codling moth.

The only currently recommended chemicals are organophosphates. Phosmet (Imidan) is preferred in an IPM program since it has lower human toxicity than azinphos-methyl (Guthion, APM, Sniper). Pyrethroids, carbaryl (Sevin), and endosulfan (Thiodan) are not recommended due to their disruptive effects on IPM programs and their short half-life.

Border or perimeter sprays can greatly reduce the pesticide load on the environment. Border sprays refer to pesticide applications applied to the outer 50 metres of orchard perimeter. This distance is sufficient to intercept any apple maggot flies, which are weak flyers, from penetrating the orchard. Border sprays are effective only when the following conditions are met:

• orchards are larger than 10 acres
• orchards are square or rectangular, not irregularly shaped
• there is no history of apple maggot or codling moth infestations in the orchard
• the orchard is not directly adjacent to any abandoned orchard or under heavy pressure from nearby wild hosts
• adequate coverage over the 50-metre-deep orchard perimeter is attainable (dependent on factors such as canopy density and spray drop size and velocity).

Additionally, orchards should be well-maintained (e.g., regular annual pruning, balanced fertilizer program).

At the headlands, nozzles on one side of the sprayer should be turned off and the spray blown into the orchard while traveling along the outer edge. Imid-an is the recommended insecticide. Imid-an generally gives 18-21 days of residual, less if frequent heavy rains occur. Subsequent sprays should be times for increases in codling moth and apple maggot activity. This information is also supplied on local fruit agri-phone updates.

A careful assessment f pest damage to fruit in the orchard should be carried out at or just prior to harvest. If an increase in injury is observed in a given year, a return to cover sprays during the summer months, should occur the following season. At that time a reassessment can be done as to whether the problem was rectified and a return to border sprays is possible.

The benefits of border sprays are: they save hundreds of dollars in spray costs; they allow populations of beneficial insects and mites to build up during the summer months; and the overall environmental impact of pesticides is reduced.

Predators such as ground beetles (Carabidae), ants and crickets, and parasitic wasps will attack larvae as they leave fallen apples, and on pupae, but do not provide economically acceptable levels of control. The removal of wild hosts like hawthorn, wild apple within 100 metres of the orchard can reduce pressure from migrating flies. Trapping out techniques, in which sticky red spheres are placed in every perimeter tree, may be effective for small orchard but has not been scientifically tested in Ontario.

To eliminate any live larvae in harvested apples, a cold storage period of eight weeks at temperature below 5°C is effective in killing them. This practice may satisfy the export restriction of some countries.

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