GardenLine | Fruit to Grow and Pick | The Mystery Of The Wormy Apples

One year, our Horticulture Information service in Saskatoon received a number of calls from gardeners complaining about brown streaks in their apples, crabapples and sometimes pears. Many callers also found tiny white maggots inside the fruit. Although calls were received from all across Saskatchewan, the majority of them came from the North Battleford area.

Problems with worms in fruit appear to be on the increase in Saskatchewan. In the past such problems were relatively rare here, though common in the other provinces.

The University of Saskatchewan's Department of Horticulture Science is asking your assistance in helping to identify the cause of the problem. At present there are three insects "under investigation."

THE SUSPECTS

Apple curculio: The curculio is a reddish brown, robust snout beetle which closely resembles the common plum curculio. Snout beetles have a large snout extending from their face. There are a number of different curculio beetles found in Saskatchewan.

Apples, crabapples and pears have been attacked by the apple curculio; ornamentals such as mountain ash have also been attacked.

Adult beetles begin laying eggs into the enlarging fruit shortly after petal fall, and continue laying eggs for up to a month. Inside the fruit, tiny white maggots (larvae) emerge from the eggs and begin tunnelling towards the core (seed cavity) of the fruit. This tunnelling leaves thin brown streaks extending from the skin in towards the core. On the skin, tiny puncture marks or dimpling can be seen where the eggs were deposited.

Once inside the core, the larvae begin feeding on the developing seeds. During summer, the larvae change into pupae. The fruit drops to the ground in autumn and with it come the pupae. Once on the ground, the life cycle is completed when the adult beetle emerges from the pupa case. The adult beetle overwinters in the soil in the near vicinity of the tree.

Apple seed chalcid: This tiny wasp is rarely seen in Saskatchewan. It is a "suspect" in our investigation because the damage it causes is similar to the damage reported by our callers. The adult wasp emerges in mid-June and immediately begins laying eggs into the enlarging fruit of apple and crabapple trees. Like the apple curculio, the larvae tunnel through the fruit aimed towards the seed cavity.

When in the core, the larvae begin feeding on the developing seeds. Brown streaks emanating from the core are typical of the damage caused by this insect. The larvae overwinter inside the fruit, which will be shed in fall. Early the following spring, the larvae change into pupae and then into the adult wasps.

Apple maggot: This pest is also called the railroad worm, because of the tunnels it makes within the fruit of apples. The adult fly is slightly smaller than a housefly and is very common across Canada. Unlike the previous two pests, these larvae tunnel throughout the flesh of the fruit, but do not eat the developing seeds.

Damage within the fruit is often extensive, with telltale tunnels rather than brown streaks. The adult fly emerges in June or July and immediately begins laying eggs into the enlarging fruit. The larvae feed inside the fruit until autumn when the fruit falls to the ground. While on the ground, the larvae burrow into the soil, pupate, and overwinter. The following spring the adult fly emerges to complete the life cycle.

CONTROL

Damage to the fruit is usually not serious, but most gardeners wish to control the pest to prevent further damage in future years. Systemic insecticides are not registered for use on fruit-producing crops like apples or crabapples and therefore they should not be used.

The most effective approach is to control the adult pest as it is laying its eggs. Contact insecticides such as ambush (permethrin), diazinon and sevin (carbaryl) should be effective. Do not spray trees when they are in flower, as this will kill the pollinating insects. Do not use sevin (carbaryl) until at least 30 days after petal fall. If applied too early, it can cause some fruit loss in apples and crabapples. Ambush (permethrin) and diazinon should be sprayed after petal fall, when the fruit is enlarging.

Another method of control is to collect and destroy all fruit that falls from the tree. The larvae or pupae of the insects fall to the ground in the fruit, and then burrow into the soil, where they overwinter. Destroying the fruit before the pest has a chance to burrow into the ground therefore should be an effective control.

HOW YOU CAN HELP

The Department of Horticulture Science would very much appreciate your assistance in helping us to determine which pest is causing the problem, so that we can provide more specific control measures. If you encounter locally grown fruit that shows brown streaking in the fruit or that contains larvae (maggots), please contact your local Extension Agrologist, who will provide mailing boxes for samples and information about sending the samples to us. With the samples, please inform us about the type of fruit, the cultivar of fruit if known, if adult pests were seen during the year, and if any control methods were tried.

Wood was an instructor with the Department of Horticulture Science. This column is provided as a service by the Extension Division and the Department of Horticulture Science, University of Saskatchewan.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.