Raspberry Crown Borer: Handle the Muncher

Growing berries at home can be a delicious and rewarding experience. Raspberries are perennial plants that send new sticks each year on which to grow fruit. Properly caring for your raspberry field and controlling pests like the raspberry crown drill are important steps to ensure you get bountiful harvests year after year.

The raspberry crown borer (Pennisetia marginata) is a species of moth that can damage many crown-forming plants, including raspberries, blackberries, and other blackberries in the Rubus family. They are not to be confused with raspberry borer (Oberea bimaculata), a type of beetle that also feeds on raspberry plants.

The raspberry crown borer is a common pest in North America and is similar to yellow jack wasps. Like many garden pests, the raspberry crown borer causes the greatest damage in the larval stage. The development of larvae tunnels and eats through the roots, sticks and crowns of plants. This blog will teach you how to identify and treat this pest before it can seriously affect your yields.

Raspberry crown drill overview

The raspberry crown borer is actually a type of moth. Source: tree growth

Adult raspberry crown borer moths have a noticeable black and yellow pattern on their body and can easily be mistaken for yellow jackets. However, these Clearwing moths have noticeable scales on their bodies and wings, while yellow jackets do not. Adults with raspberry burrs, unlike yellow jackets, also have yellow legs and feathered antennae. They fly during the day and are most active in late summer. From July to September, the females lay individual brown oval eggs on the underside of the leaves. These eggs are approximately 1/16 long and any adult female with a raspberry crown borer can lay up to 140 eggs per season.

Life cycle of a raspberry crown drill

The raspberry crown drill typically has a life cycle of two years in North America. One to two months after the eggs are laid in late summer, the eggs hatch and small larvae crawl over the hives of the host plants. They then form bubble-like hibernacula on the base of the canes or hide in protected areas under the bark to hibernate. These new larvae are inactive until next spring, when they begin to tunnel through their host and feed on the first spring and summer.

The larvae overwinter in the plant roots and continue to feed in the second year. They will be fully grown by midsummer of their sophomore year and be 1 to 1.5 inches in length. At this point, they are fleshy cream-colored maggots with dark brown heads. After a short pupation period of 2-3 weeks, these larvae appear as adult moths to restart the cycle.

Common living spaces

Raspberry crown drills depend on their host plants for every part of their life cycle. The eggs can be spotted on raspberry and blackberry leaves, and the larvae reside in the roots, crowns and sticks of their host plants. In late summer, inspect the underside of your raspberry leaves for eggs.

Other signs that you may have a problem with the raspberry crown borer may include a general decrease in your plant's vitality and production, dying or wilting leaves, broken sticks, and holes in the crown and upper roots of the plant. You can also look for fretted or cast parts around the plant crowns and roots, which have a sawdust-like appearance. As the larvae grow and tunnel, they can also swell at the base of the sticks.

What do Borers eat?

Raspberry crown borer feed on plant tissue in the larval stage. Feeding them disrupts the transport of nutrients and water from the root system through the canes, causing the appearance of wilts and an overall decrease in the vigor and production of the plants.

How to control raspberry crown drills

Front view of the raspberry drill mothThis humble moth can do great harm to berry plants. Source: System Layers

Raspberry crown drills are most common in eastern North America and the Pacific Northwest. While they won't kill your raspberry or blackberry plants right away, this pest problem can spread from plant to plant and from snowball if left untreated.

Organic or chemical control

Raspberry crownworm control insecticides are only available for commercial use and are applied in the fall or before flowers begin to form in spring. These are controlled pesticides that target first year larvae that feed on plant roots.

There are only very limited opportunities for organic farms or self-farmers. However, you can check your plants for signs of adult moths and new eggs throughout the growing season. Raspberry plants have vigorous annual growth and pruning is an important part of raspberry care, both for productivity and for pest control.

When new canes appear in May, check for signs of infestation and prune as needed. During the growing season, watch out for withering and dead sticks that may have larvae in them and remove them immediately. At the end of the fruiting season, prune the canes again and look for signs of drill damage. This practice should help not only with the raspberry crown drills, but likely other pests that afflict raspberries as well.

Environmental control

One of the most effective ways to control raspberry crown bits for home gardeners is to remove affected crowns and destroy them before the next season. If you notice swelling on the plant sticks from burs, be sure to prune underneath the swollen area. Also, remove any wild blackberries in the area to limit alternative hosting options for this pest.

There have also been some studies of the use of nematodes such as Steinernemafeltiae, Steinernema carpocapsae, and Heterorhabditis bacteriophora against raspberry crown borer. The use of nematodes has shown some positive results in reducing the total number of larvae.

frequently asked Questions

Raspberry crown mothRaspberry crown borer moths look a lot like wasps with no quills. Source: tree growth

Q: How do you stop pipe drills?

A: Since raspberry crown drills typically have a two year life cycle, the best way to control this pest is to prune the affected sticks to ensure the larvae cannot mature into adulthood in the first year.

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