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. Plum fruits will sometimes produce a clear liquid (gumming), this is thought to be a physiological response to changing water availability and not related to the presence of the moth
• 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
• Affected 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. This often goes unnoticed as overall yield is often unaffected. 

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

Control

Use pheromone traps placed in May (details below), so if necessary action can be taken before a damaging infestation has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce the damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.

 
Non-pesticide control

• Where possible tolerate the loss of some fruit to plum moth, often only a small proportion of fruits are affected. At first levels of the moth can seem high as the first fruit to ripen are those that have been affected by caterpillars
• Encourage predators and other natural enemies in the garden such as birds, hedgehogs and ground beetles

• Pheromone traps can be used to trap male moths. They use a synthetic version of volatile chemical (sex-pheromone) produced by female moths to lure in males
• 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. Male plum moths are lured into the trap and get stuck
• These traps alone rarely control plum moth but monitor moth activity. This can improve control by indicating if a pesticide spray is worthwhile and can help accurately time pesticide use if deemed necessary
• On isolated trees these traps may catch enough males to reduce the females' mating success, resulting in fewer fertile eggs being laid
• By counting the trapped males every week and following the instructions that come with the trap, the best time to spray can be calculated
• Pheromone traps are designed to prevent birds entering the trap and getting caught on the sticky sheet, although occasionally birds can enter the traps and it may be necessary to add additional bird netting  

• Plum moth caterpillars can only be controlled on apple and pear with insecticides before they enter the fruits
• Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer). Several applications of these short persistence products may be necessary to give good control
• More persistent contact insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
• 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. In some years, egg hatching may be earlier or later, due to the weather conditions and in years with long summers there may be two generations of the moth 

Follow label instructions when using pesticides. On edible plants make sure the food plant is listed on the label and follow instructions on maximum number applications, spray interval and harvest interval.
Plants in flower should not be sprayed due to the danger to bees and other 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 (pdf document)

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.