What is plum moth?

Plum moth has pinkish caterpillars that feed inside the ripening fruits of plums, damsons and gages. 

Symptoms

Indications of a plum moth include;

• Fruits with a caterpillar inside usually ripen early and are often slightly misshapen. This should not be confused with pocket plum, a fungal disease affecting plum fruits 
• The caterpillar inside the fruit is up to 12mm long and is pale pink with a brown head
• There will be many light brown excrement pellets near the plum stone where the caterpillar has been feeding
• Infested fruits tend to ripen first, fruits that ripen later on the tree often have a much lower incidence of the caterpillars

A resinous gum around the stone is a physiological disorder and should not be confused with plum moth damage.

Plum sawfly

Plum sawfly is a less frequent on plums than plum moth. The larvae tunnel into three or four fruitlets before going into the soil to pupate. Unlike plum moth whose caterpillar develop in the mature fruit, fruitlets damaged by plum sawfly fall from the tree at an early stage in June.

The plum cultivars ‘Czar’ and ‘Victoria’ seem to be more susceptible than others to plum sawfly.

Control

Pheromone traps

• Pheromone traps for plum moth are available from garden shops or from some mail order suppliers of pest controls or biological pest controls. These consist of an open-sided box that is hung in the tree in early May. The bottom of the box has a sticky sheet on which the pheromone pellet is placed. The pheromone is synthetic copy of a volatile compound produced by female plum moths to attract mates. Male plum moths are lured into the trap and get stuck
• Pheromone traps alone rarely control plum moths, but on isolated trees may catch enough males to reduce the females' mating success, resulting in fewer fertile eggs being laid
• Pheromone traps can be used to more accurately time pesticide sprays against this insect 
• By counting the trapped males every week and following the instructions that come with the trap, the best time to spray can be calculated
• Occasionally birds can enter the traps and it may sometimes be necessary to add a bird net cover to the trap entrances to stop this

Pesticide control

• On trees small enough to be sprayed, the newly-hatched caterpillars can be killed by using the contact synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Pest Killer) or deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer)
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available
• Timing of spray can be more accurately determined by the use of a pheromone trap (see above) otherwise a spray in about the third week of June, with a second application about three weeks later may have some effect
• Manufactures instructions for pesticides on maximum dose, number of applications and harvest interval must be followed for food crops
• The sprays will also give some control of plum sawfly if applied just after petal fall
• Plants in flower should not be sprayed due to the danger to pollinating insects
• Inclusion of a pesticide product does not indicate a recommendation or endorsement by  RHS Gardening Advice. It is a list of products currently available to the home gardener

Download

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biology

Plum moth usually has one generation a year with adult moths emerging from late May onwards, but mostly in mid-June to mid-July. The newly hatched caterpillars tunnel into the fruits and feed around the stone until late summer. When fully fed, they emerge and overwinter inside silk cocoons spun under loose bark or other concealed places.

In warm summers, some caterpillars may pupate early and produce a second generation in late summer.