Cranberry fruitworms are a serious pest of blueberries, cranberries, and cherries . Infested berries may be harvested without detection, resulting in worms being found in packaged berries, not a pleasant sight when you've already eaten a few - good protein though.
The adults are small brownish nocturnal moths that emerge in early spring and lay their eggs on unripe berries.
The larvae, when they hatch will feed inside the fruit and migrate from berry to berry as they mature. Cranberry fruitworm larvae produce masses of silky webbing as they feed in berry clusters, similar to cherry fruitworm larvae which will bore directly from one fruit to another and seal the connecting points with silk. The cranberry fruitworm larvae are not as red as the cherry fruitworm.
Infestations are not easily detected early in the season as there is not much external evidence of this pests presence. Inspect your plants early in the season and look for pin-sized entry holes near the base of berries. Berries may slightly off color, cranberries sometimes turn blue. Open neighboring berries to seek out the larva.
The frass and webbing cranberry fruitworms produce will generally appear when they are several weeks old and begin migrating from berry to berry.
The larva [worm] is a smooth pale yellow-green elongated caterpillar with an amber head, slightly over an inch long. Larvae feast inside berries until most contents are fully consumed. The berry shrivels and discolors.
There is frequently "frass" around damaged berries. Frass is their excrement.
The cherry fruitworm, a similar species which infests the same host leaves only a lining of silk at the meeting points of berries within clusters . Most larvae are fully grown by late spring. Larvae will then drop to the soil surface and build a silken chamber.
Prevention and Control
Eliminate plant debris and weeds around plants to help by reducing overwintering protection for fruitworm cocoons. Diverse gardens that include a symbiotic mingling of flowers and vegetables are a poor environment for insect pests as they also attract many natural predators, Lacewings and Lady Bugs in particular. Parasitic wasps sometimes attack the fruitworms, but are not exceptionally effective as the fruitworm is inside the fruit itself . The Praying Mantis will prey on the moths. Adult moths can be trapped with light traps by night.
Hand picking of infested berries is feasible, as stated look for the webbing and premature ripening and discoloration. This method is very practical in home gardens with light infestations. In heavy infestations insecticide may be necessary to achieve satisfactory control.
Bt Bacillius thuringiensis should be applied at either bloom or petal fall, or both. Bt is not generally harmful to beneficial insects. It is a stomach poison and must be ingested by the caterpillar and is effective when applied during warm, dry weather while the larvae are actively feeding. Requires more than 1 treatment
Bio Pesticides such as Neem derived products and Spinosad , another biopesticide , harmless to humans are advisable. Spinosad will interfere with pollinators as well as pests - evening / night time use is advisable. Insecticides, such as Pyrethrin or malathion only as a last resort.
Sprays/Pesticides must be applied in the pre-bloom stage to prevent injury. Pesticides such as pyrethroids that are more effective in cool than warm weather will work more efficiently against Fruitworms earlier in the season, and is the recommended early season chemical treatment for most Fruitworms.
Proper sanitary practices are vital to the health of your garden. Proper sanitation can help to ensure disease-free pest-free and productive gardens. Try to keep the garden free of any diseased dead or damaged plant materials. Remove cuttings from pruning and trimming and either destroy them - if diseased, or send them to the compost pile. Leaving rotting fruits and vegetables in the garden is like a written invitation to unwanted pests and diseases.
If a diseased or dead plant part has to be cut, the microorganism that caused the problem is probably on the tool you just used. Like a surgeon, sterilize all tools by washing in soap and water - rubbing alcohol wouldn't hurt either. If you pinch off diseased plant parts, wash hands before handling any other plants. Keep Weeds under control. Till the soil in the spring before planting to expose and kill larvae that wintered over in the soil.