Raspberry Fruitworms

Also known as the Western raspberry fruitworm, Byturus unicolor is the US counterpart to the north-central European raspberry beetle (B. tomentosus). The American raspberry beetle is something else entirely.

As their name suggests, these pests will feed on your raspberries, but boysenberries, loganberries, salmonberries, and thimbleberries are not exempt. Raspberry fruitworms are found throughout Central and North America.
 
To be honest, fruitworm beetles don’t look like much of a threat to anything, but their offspring can be something else altogether.

Raspberry fruitworm beetle (Robert Webster) CC BY-SA 4.0

Raspberry fruitworm identification
 
Adult fruitworm beetles are only 0.16 to 0.19 inches long (4-5mm), so you could fit three or four of them across the face of an American dime. They are yellowish-brown with tiny hairs.

Raspberry fruitworm larvae are twice as long as their parents. They are white underneath with light brown segments on top and darker brown lines along the center.
 
Fruitworm larvae get bigger than their parents by eating your young raspberries. But the problem starts much earlier than that.
 
Raspberry fruitworm lifecycle
 
Adult fruitworm beetles spend the winter underground or hidden under leaves, protected against the elements. In spring, they emerge and start eating young raspberry leaves and flower buds. They start close to the ground and work their way up the canes to be closer to developing flower buds. Fruitworm beetles prefer feeding on the tissue found between the veins of new, unfolded leaves. This skeletonization is a clue.
 
After mating, females lay more than 100 eggs on and near developing flower buds. Eggs hatch about the same time fruit is developing. The larvae burrow into the fruit, out of sight and safe from predators, where they feed for a month or so before dropping to the ground to pupate in the soil and begin the cycle again.
 
All this feeding and burrowing can leave your raspberry harvest looking pretty meager. And there’s no guarantee that that raspberry you’re about to pop in your mouth doesn’t contain a guest. Ew!
 
Raspberry fruitworm management
 
The first step in reducing fruitworm damage is to monitor your plants regularly, as soon as new growth starts in the spring. Look for signs of adult and larval feeding. Handpicking adult beetles before they start laying eggs is much easier than dealing with a bigger problem later on. These beetles are most active in early evening, so that’s the best time to look for them. After removing beetles from your plants, simply drop them in a container of soapy water for chemical-free control. You can also use sticky traps near your raspberry plants to monitor for these and other pests.
 
Ladybugs and lacewings are your allies in this battle, so make sure your garden is welcoming and free from indiscriminate chemicals. Fruitworms are more of a problem when weeds and wild blackberries are present, so keeping the area around your canes tidy and clean can help prevent the problem.
 
Serious infestations may call for applications of spinosad or Bacillus thuringiensis (Bt). There are several types of Bt. To deal with fruitworms, you will need BtK, which can be found at local garden supply stores. This is best done during warm evenings while the plants are in the flowering stage. Follow package directions carefully and do not spray open flowers.
 
After harvesting your raspberry crop, rake the area around the canes lightly to make life more difficult for overwintering beetles. If you have chickens, let them have the run of the area for a few days. Hens can be powerful pest control, and they are fun to watch.