Growing berries at home can be a delicious and rewarding experience. Raspberries are perennial plants that send up new canes every year on which fruits grow. Properly caring for your raspberry patch and managing pests like the raspberry crown borer are important steps to making sure you get abundant harvests year after year.
The raspberry crown borer (Pennisetia marginata) is a type of moth that can damage many crown-forming plants, including raspberries, blackberries, and other brambles in the Rubus family. They are not to be confused with raspberry cane borers (Oberea bimaculata), which are a type of beetle that also feed on raspberry plants.
The raspberry crown borer is a common pest in North America and resembles yellow jacket wasps. Like many garden pests, the raspberry crown borer causes the most harm during the larval stage. Developing larvae will tunnel and eat through the roots, canes, and crowns of plants. This blog will cover how to recognize and treat this pest before it can severely impact your yields.
Raspberry Crown Borer Overview
Adult raspberry crown borer moths have a striking black and yellow pattern on their bodies and can be easily mistaken for yellow jackets. However, these clearwing moths have noticeable scales on the body and wings whereas yellowjackets do not. Raspberry crown borer adults also have yellow legs and feathery antennae, unlike yellowjackets. They fly during the day and are the most active in late summer. During July through September, females will lay individual brown oval eggs on the underside of leaves. These eggs are roughly 1/16 long and each raspberry crown borer adult female can lay up to 140 eggs per season.
Life Cycle of A Raspberry Crown Borer
The raspberry crown borer typically has a two-year life cycle in North America. One to two months after the eggs are laid in late summer, eggs will hatch and small larvae will crawl down the canes of host plants. They will then create blister-like hibernacula on the base of the canes or hide in protected areas under the bark to overwinter. These new larvae will be inactive until the following spring when they start to tunnel and eat through their host throughout their first spring and summer.
The larvae will overwinter in the plant roots and continue feeding during their second year. They will be fully grown around mid-summer of their second year, measuring 1-1.5 inches in length. At this point, they will be fleshy cream-colored grubs with dark brown heads. After a brief pupation period of 2-3 weeks, these larvae will emerge as adult moths to restart the cycle.
Raspberry crown borers depend on their host plants during every part of their life cycle. The eggs can be spotted on raspberry and blackberry leaves and the larvae can be found in the roots, crowns, and canes of their host plants. In late summer, inspect the underside of your raspberry leaves to check for eggs.
Other signs that you may have a raspberry crown borer problem can be an overall decrease in the vigor and production of your plant, dying or wilting leaves, canes that have broken off at the grown and holes in the crown and upper roots of the plant. You can also check for frass, or castings, around the plant crowns and roots which will have a sawdust-like appearance. As the larvae grow and tunnel, they might also cause swelling at the base of the canes.
What Do Borers Eat?
During their larvae stage, raspberry crown borers feed on plant tissue. Their feeding disrupts the transport of nutrients and water from the root system up through the canes which is what causes the appearance of wilting and an overall decrease in the plants’ vigor and production.
How to Control Raspberry Crown Borers
Raspberry crown borers are most commonly found in the eastern part of North America and parts of the Pacific Northwest. While they won’t kill your raspberry or blackberry plants immediately, this pest problem can spread from plant to plant and snowball if not managed.
Organic or Chemical Control
Insecticides to control raspberry crown borers are only available for commercial use and are applied in the fall or before flowers form in the spring. These are controlled pesticides designed to target first-year larvae that are feeding on plant roots.
There are very limited options for organic operations or home growers. However, you can inspect your plants throughout their growing season to look out for signs of adult moths and new eggs. Raspberry plants have vigorous annual growth and pruning is an important part of raspberry maintenance both for productivity and pest management.
As new canes emerge in May, check them for any signs of infestation and prune as needed. During the growing season, look out for wilting and dead canes which might be harboring larvae and remove them immediately. At the end of the fruiting season, prune canes again and look out for signs of borer damage. This practice should help not only with the raspberry crown borers, but likely other pests that plague raspberries as well.
One of the most effective ways to control raspberry crown borers for home gardeners is to remove affected crowns and destroy them before the next season. If you spot swelling on the plant canes from borers, make sure to prune beneath the swollen area. Also, remove all wild brambles in the area to cut down on alternative host options for this pest.
There have also been some studies of using nematodes, such as Steinernemafeltiae, Steinernema carpocapsae, and Heterorhabditis bacteriophora, against raspberry crown borers. The use of nematodes has shown some positive results in decreasing the overall number of larvae.
Frequently Asked Questions
Q: How do you stop cane borers?
A: Since raspberry crown borers typically have a two-year lifecycle, the best way to manage this pest is by pruning the affected canes to make sure first-year larvae cannot mature to adulthood.