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Raspberry Fruitworm: Fruit-Feeding Larvae

Berries are some of the tastiest treats that you can grow in the garden. Cultivating raspberries is not too difficult as long as you can control the trifecta of pests that commonly affect them: raspberry cane borer, raspberry beetle, and raspberry fruitworm.

The raspberry fruitworm (Byturus unicolor) is the larval stage of a small brown beetle that attacks raspberry plants. Despite its name, this pest is not unique to just raspberries. They can also feed on loganberries, boysenberries, wild blackberries, salmonberries, and thimbleberries. 

Raspberry fruitworm is a major pest for farmers who cultivate the raspberry fruit commercially. They not only decrease the overall yield of the crops from damage to flower buds, but they also contaminate fruit and make them unfit to be sold at markets. 

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Raspberry Fruitworm Overview

Raspberry fruitworm beetle
The adult beetle form of the raspberry fruitworm. Source: treegrow

The history of the raspberry fruitworm in North America is well documented with the earliest records dating back to the late 1800s. These records show that the raspberry fruitworms have historically been plaguing berry producers in the northern US and Canada. The raspberry fruitworm is also known as the western raspberry fruitworm because of its prevalence in western parts of the continent such as Washington state and the province of British Columbia. 

The raspberry fruitworm develops into beetles that are small (1/4″) and reddish-brown in color with tiny hairs covering the whole body. These hairs are a distinguishing trait of the species. Female beetles lay small eggs on flowers or green fruits. Eggs hatch into small whitish-yellow grubs or fruitworms. Although the raspberry fruitworm Byturus unicolor develops into a beetle, this species is related to but distinct from the raspberry beetle, Byturus tomentosus

Life Cycle of Raspberry Fruitworms

Adult beetles overwinter in the soil and emerge in early spring, between mid-April through mid-May, to begin feeding on leaves close to the ground. They will move up the canes and are attracted to flower buds. Adult beetles will mate and adult females will lay their eggs on these flower buds and immature fruits. When the eggs hatch, the small larvae have direct access to the raspberry fruit and will continue to feed until the fruit is ripe in late summer. The larval stage typically takes 30-40 days. Fruitworms will pupate and become full adult beetles in the autumn after mature fruits drop to the ground. 

Common Habitats

Adult raspberry fruitworm beetles overwinter in the soil and under leaf litter from the autumn through early spring. They emerge in early spring to feed and mate on cane plants, working their way up the plant from the bottom leaves to the new primocanes, floricanes, and flower buds. Their larvae are typically found within the developing raspberry fruit. 

What Do Raspberry Fruitworms Eat?

The raspberry fruitworm attacks the living tissue of raspberry plants and is most active in the spring and summer. Adults feed on the young leaves and flower buds and will leave an interveinal pattern of damage on raspberry leaves. This means that you can spot strips of chew-marks between the veins of the leaves. This skeletonized leaf damage pattern can help you diagnose this pest issue. The fruitworm is typically found inside the berry and bores the developing fruit from the inside out. 

How to Control Raspberry Fruitworms

Raspberry fruitworm larvae
A raspberry fruitworm larvae. Source: OSU Ken Grey Insect Image Collection

The best time to manage raspberry fruitworms is during the adult stage before they have a chance to lay eggs. The fruitworms are so small and live within the fruit that it becomes time-consuming and tedious to handpick them at that stage. Many commercial growers are also not able to do handpick worms if they use machines to harvest fruits. 

Organic or Chemical Control

Stay vigilant and monitor raspberry plants for these beetles in early spring. Inspect lower leaves for any damages and use sticky traps near the raspberry plants to monitor their population.  You can try to handpick off the beetles and drop them into some soapy water to kill them without the use of chemicals. 

If you must use chemical controls, you can try treating your plants with spinosad, an organically approved microbial pesticide, or with pyrethrin, another organic spray derived from chrysanthemum flowers. Follow best practices to ensure that your use of chemicals is not harming bees. Spray the canes when there is noticeable beetle activity and again when the flowers start to form and flower clusters separate. Raspberry fruitworm beetles are most active on warm evenings so spraying during these conditions might yield the best results. Do not spray when the flowers open since these flowers will start to attract pollinators. 

Environmental Control

Some studies have shown the raspberry fruitworm problem to be more prevalent where there are more weeds. Remove weeds and debris around raspberry plants to remove possible shelter for overwintering adult beetles. There are several wild Rubus, or brambles that can be hosts to raspberry fruitworms. Remove these wild hosts like thimbleberries and blackberries around your raspberries as a way to isolate the fruitworm population. 

Preventing Raspberry Fruitworms

After the raspberry harvest, lightly rake the area around each plant to break up or kill any overwintering adult beetles. If you are raising chickens, you can release them to feed around the raspberry canes and they can eat the overwintering adult beetles before they emerge the following spring. 

Frequently Asked Questions

Raspberry fruitworm adult damage
Damage caused by the raspberry fruitworm beetle. Source: wit

Q: Are raspberry worms harmful to humans?

A: Raspberry worms are not known to cause damage to humans when ingested. However, they cause a lot of economic damage to farmers because they contaminate fruits and cause them to not be fit for sale. 

Q: Are there worms in raspberries?

A: There can be multiple types of worms found in raspberries including the raspberry fruitworm and also the larvae of spotted wing drosophila, Drosophila suzukii, a type of vinegar fly.


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