How to Identify and Control Raspberry Crown Borer
The raspberry crown borer can do serious damage to all sorts of berry plants. Sustainability specialist Huan Song explains how to identify and control these damaging pests and save your harvests.
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Growing berries at home can be a delicious and rewarding experience. But properly caring for your raspberry patch and managing pests like raspberry crown borer are essential to ensure you get these abundant harvests year after year.
The raspberry crown borer (Pennisetia marginata) is a type of moth that damages many crown-forming plants. This includes raspberries, blackberries, and other brambles in the Rubus family. They are not to be confused with raspberry cane borers (Oberea bimaculata), a type of beetle that also feeds on raspberry plants.
The raspberry crown borer is a common pest in North America and resembles a yellow jacket wasp. Like many garden pests, it causes the most harm during the larval stage. Learn how to recognize and treat this pest before it severely impacts your yields.
Identifying Raspberry Crown Borer
Adult raspberry crown borer moths have a striking black and yellow pattern on their bodies. This coloration means they are easily mistaken for yellow jackets.
However, these clearwing moths have noticeable scales on their body and wings, while yellowjackets do not. Raspberry crown borer adults also have yellow legs and feathery antennae, unlike yellow jackets.
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 one-sixteenth of an inch long. Each raspberry crown borer adult female can lay up to 140 eggs per season.
Life Cycle
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. This is when they start to tunnel and eat through their host throughout their first spring and summer.
The larvae will overwinter in plant roots and continue feeding during their second year. They will be fully grown around mid-summer of their second year, measuring one to one and a half inches in length.
At this point, they will be fleshy cream-colored grubs with dark brown heads. After a brief pupation period of two to three weeks, these larvae emerge as adult moths to restart the cycle.
Common Habitats
Raspberry crown borers depend on their host plants during every part of their life cycle. The eggs appear on raspberry and blackberry leaves and the larvae in the roots, crowns, and canes of 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 are:
- Overall decrease in the vigor and production of your plant.
- Dying or wilting leaves.
- Canes broken off at the crown.
- Holes in the crown and upper roots of the plant.
Also check for frass, or castings, around the plant crowns and roots which 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 through the canes. This 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 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 raspberry crown borer infestation and prune as needed. During the growing season, look out for wilting and dead canes that 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 will help not only with the raspberry crown borers but other pests that plague raspberries too.
Environmental Control
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, prune beneath the swollen area. Remove all wild brambles in the area to cut down on alternative host options for this pest.
There have also been some studies using nematodes, such as Heterorhabditis bacteriophora, against raspberry crown borers. The use of nematodes has shown some positive results in decreasing the overall number of larvae.
To apply an effective treatment, wait for an expected period of temperate weather in spring or fall. The soil should be no colder than 42°F and no warmer than 95°F (6-35°C). Apply two treatments two weeks apart. This should eliminate any overwintering larvae.
