How to Identify, Control, and Prevent Raspberry Fruitworm

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. These include raspberry crown 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. This makes them unfit to be sold at markets. 

What is Raspberry Fruitworm?

In North America, this pest 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. It is also known as the western raspberry fruitworm because of its prevalence in western parts of the continent. These include states such as Washington and the province of British Columbia. 

This pest develops into beetles that are a quarter of an inch and reddish-brown in color. They will have 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 this pest develops into a beetle, it is also related to but distinct from the raspberry beetle, Byturus tomentosus. 

Life Cycle

Adult beetles overwinter in the soil and emerge in early spring. Between mid-April through mid-May, they begin feeding on leaves close to the ground. They 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. They 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. In early spring they feed and mate on cane plants. They will work their way up the plant from the bottom leaves up to the 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. 

Control

The best time to manage raspberry fruitworms is during the adult stage before they have a chance to lay eggs. They are so small and live within the fruit, which makes it time-consuming and tedious to handpick them at that stage. Many commercial growers are also not able to 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 damage and use sticky traps near the raspberry plants to monitor their population. Handpick any beetles and drop them into some soapy water to kill them without the use of chemicals. 

If using chemical controls, treat your plants with spinosad, an organically approved microbial pesticide, or with pyrethrin. Follow best practices to ensure that your use of chemicals is not harming bees.

Spray the canes when there is noticeable beetle activity. Spray 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. 

Prevention

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, release them to feed around the raspberry canes. They can eat the overwintering adult beetles before they emerge the following spring. 

Frequently Asked Questions