Why Are My Plums Full of Maggots?

Cutting open a plum to find a pinkish-white caterpillar surrounded by dark frass inside is one of the most common surprises for plum growers in late summer. The culprit is almost invariably the plum moth (Grapholita funebrana), a small tortrix moth whose larvae develop inside the fruit from mid-June onward. Understanding the moth's life cycle is the key to protecting your crop.

The plum moth life cycle

Adult plum moths emerge from overwintering pupae in soil and bark from late May onward. Females lay single eggs on developing fruitlets. Hatching larvae burrow through the skin and feed their way to the stone, eating the flesh around it and leaving behind a mass of dark frass. Infested fruits often ripen early and drop from the tree in July and August before the rest of the crop. The larva exits the fallen fruit, drops to the ground and pupates in soil or bark crevices to overwinter. One to two generations can occur per season in warm summers.

Pheromone trap monitoring

Sticky pheromone traps baited with synthetic female sex pheromone are the most useful management tool for the home grower. Hang one trap per tree from late May. Count catches twice weekly — when trap catches spike, adult moths are flying and egg-laying is at its peak. This tells you the precise window for any intervention. Traps alone do not control the population (they catch only males) but they time other actions accurately.

Removing infested fruit to reduce overwintering population

Collect and destroy all dropped fruits immediately throughout summer — do not leave them on the ground or compost them as the larvae inside will survive and add to next year's population. Shake the tree gently in July and August to bring down fruits harbouring larvae. This alone can significantly reduce infestations over successive seasons.

Biological control with nematodes

A soil drench of Steinernema carpocapsae nematodes applied in late summer attacks pupating larvae in the soil beneath the tree. Apply when soil temperature is above 14°C, keeping the soil moist for two weeks afterward. This approach reduces the number of adults emerging the following season and fits well into an organic management programme.

Insecticide options

Pyrethrin-based sprays applied at the time of peak moth flight (as indicated by pheromone trap catches) target newly hatched larvae before they enter the fruit. Timing is critical — once inside, larvae are protected. Repeat applications may be needed if the trap shows two flight peaks.