Pests commonly referred to as “worms” in fruit are actually the larval stage of various insects. The most common culprits include the caterpillars of moths, such as the codling moth and oriental fruit moth, and the legless maggots of flies, like the apple maggot fly. These larvae cause damage by boring into the developing fruit, which often results in premature dropping or fruit that is inedible at harvest. Effective management requires a combination of strategies that interrupt the pest’s life cycle before the fruit is compromised.
Cultural and Physical Prevention
Proper pruning creates an open canopy, which increases air circulation and sunlight penetration, making the tree less appealing to egg-laying moths and flies. This improved airflow also allows any applied treatments to cover the fruit and foliage more thoroughly.
Sanitation prevents the overwintering of pests. It is important to promptly remove and destroy all dropped fruit throughout the season, as this fruit often contains mature larvae preparing to burrow into the soil for pupation. Fallen fruit should be disposed of in the trash or sealed in a black plastic bag and left in the sun to kill the larvae, rather than being added to a home compost pile.
Physical barriers can be applied directly to the tree or the fruit to block access by adult insects. Wrapping bands of corrugated cardboard around the trunk can trap codling moth larvae seeking protected spots to spin their cocoons. These bands must be regularly removed and destroyed to ensure the trapped pests do not complete their life cycle.
Exclusion barriers offer a non-chemical solution for trees with a manageable number of fruits. Small fruit, when they are roughly the size of a nickel or quarter, can be slipped into nylon orchard sox or plastic sandwich bags with the bottom corners clipped for drainage. This physical barrier prevents adult insects, such as the apple maggot fly, from laying eggs on the fruit’s surface.
Specialized traps monitor pest populations. Pheromone traps, which contain a lure mimicking the female sex pheromone, are highly specific and only capture male moths, helping to signal the start of a pest’s flight period. For apple maggot flies, sticky red spheres or yellow panels baited with a scent like ammonium carbonate are used to trap the adults.
Timing Prevention Efforts
The success of any treatment relies heavily on targeting the pest at its most susceptible life stage, which is typically the egg or the newly hatched larva before it bores into the fruit. This precise timing prevents the larva from finding protection inside the developing fruit, where most treatments cannot reach them.
Dormant oil is applied before the buds begin to swell in early spring. This oil is designed to smother overwintering pests, such as scale insects and mite eggs, that are hiding in the bark crevices and on the branches. This early treatment reduces the overall population pressure before the growing season even begins.
For pests like the codling moth and apple maggot, the most critical timing for intervention is immediately after petal fall, which marks the transition from flower to fruit development. Treatments must be applied during this period and repeated to coincide with the hatching of successive generations of eggs. Monitoring traps help determine the start of a pest flight, and then calculations based on accumulated heat units, known as degree-days, predict when egg hatch will occur.
Targeted Treatment Solutions
When cultural and physical methods are insufficient, specific treatments can be applied to protect the crop. Treatments are most effective when they target the narrow window between egg laying and larval entry into the fruit.
Bacillus thuringiensis (Bt) is a biological insecticide that is effective against the caterpillars of moths, such as the codling moth and green fruitworm. This naturally occurring bacterium must be ingested by the caterpillar to be effective, where it releases a toxin that paralyzes the insect’s digestive system. Because Bt is quickly degraded by sunlight, it requires repeated applications, often weekly, to maintain coverage and target newly hatched larvae.
Spinosad is a low-toxicity option effective against many fruit pests. It can be used against codling moth and apple maggot, but its application must be timed carefully to avoid contact with active pollinators, as it is toxic to bees for a few hours after drying. Horticultural oil, a lighter version of dormant oil, can be added to both Bt and spinosad applications to improve coverage and increase their effectiveness.
Kaolin clay, a fine white powder mixed with water, acts as a physical repellent. The white coating deters adult apple maggot flies from laying their eggs because the fruit no longer looks like a desirable host. This material must be reapplied frequently, especially after rain or as the fruit expands, to maintain a complete barrier.
Always follow the instructions on product labels precisely, especially concerning the pre-harvest interval. Furthermore, any spraying of insecticides should be avoided during the bloom period to protect honeybees and other pollinators, which are necessary for fruit set.