WSU Whatcom County Extension

Integrated Pest Management for Raspberries

Raspberry Crown Borer

(Pennisetia marginata)

Insects and Invertebrates

 

Raspberry Crown Borer Larva

 

Symptoms

The larva of the Raspberry Crown Borer (RCB) feed on crowns, at the base of canes, and larger roots resulting in reduced yields and plant death. Symptoms include canes prematurely dying, spindly cane growth and reduced leaf size. A hole at the base of the plant with sawdust-like frass at the entrance, swelling at the base of canes, or tunnels in the canes that are noticed while pruning are a good indication RCB is present in your field. Canes may also break easily from the base during winter tying.

 

Identification

The day flying adult is a clear-winged moth resembling a yellow jacket wasp in color and size. It has a wingspan of about 1 inch and has a long, black body with four yellow horizontal stripes on the abdomen as well as stripes on the thorax. The legs are yellow, and the feathery antennae curve outward from the head, unlike the short antennae of a yellow jacket wasp.

Eggs are reddish brown and usually found on the underside edges of leaflets in late summer. The larvae are white with brown heads, have six short legs and are found in tunnels inside canes, crowns or around the root structure. The overwintering first year larvae are about ¼ inch long. Mature larvae, found in the second year of their life cycle, range from 1 to 1 ½ inches long.

 

Raspberry Crown Borer Adult

 

Life History

The raspberry crown borer takes two years to complete its life cycle. Female moths lay up to 140 reddish brown eggs, which are most often found on the underside edges of leaflets in August or September. Once hatched (September or October), larvae migrate to the crown of the plant where they either dig into the base of the cane and form a blister-like hibernaculum (hibernation structure) or find a protected area under the bark of the crown to overwinter. In the spring, the larvae feed on cane buds and burrow galleries through the crown of the plant. The feeding causes swelling or galls to form at or below the soil surface and continues through the summer. The larvae overwinter the second year in the roots or tunnels bore into the crown. In the spring the larvae continue feeding in the roots and crown, and become fully grown in late June to July. The pupation stage occurs in a short period of two to three weeks. Adult moths emerge in August and are active until late September. Considering this insect requires two years to complete its life cycle, both large and small larvae can be found infesting canes in April and May.

 

Monitoring

Monitoring for this pest is difficult because it is hidden away in the crown of the plant. Scout 3-5 sites per field (depending on field size) and evaluate 10-20 hills, spaced 3-5 hills apart. Record the percentage of infested canes at each site. In winter or spring, watch for canes that break off when tying up and search at the base of cane buds for young larvae. Monitor for swelling or galls forming at or below the soil surface which is caused by caterpillar feeding. In the fall, check lower canes and crown material closely in weak areas of the field for first or second year caterpillars, particularly if adult crown borers were seen during the late summer months.

A pheromone is available for this pest but has been ineffective as a monitoring tool. Populations can increase rapidly; therefore evidence of crown borer damage should not be ignored.

 

Raspberry Crown Borer

 

Thresholds and Management

If more than 5% of the plants have damage caused by borer larvae feeding, chemical controls are recommended. Insecticide application must be carefully timed and thorough. Follow treatment with irrigation or preceding a rain to thoroughly drench the soil. Because the insect has a 2-year life cycle, it may be necessary to treat in the spring and fall for two or more successive years in order to obtain complete control.

Removal of wild blackberries from field borders can help reduce populations and possibly prevent infestations from recurring. Infested canes should be identified, removed and destroyed.

 

Resources

Oregon State University: Integrated Plant Protection Center, Raspberry Crown Borer,
http://uspest.org/pdf/reb79.pdf

University of California, UC IPM Online, Caneberries: Raspberry Crown Borer, http://www.ipm.ucdavis.edu/PMG/r71300511.html

University of Kentucky Cooperative Extension, Raspberry Crown Borer, http://www.ca.uky.edu/entomology/entfacts/entfactpdf/ef226.pdf

Washington State University, Small Fruit Pests: Biology, Diagnosis and Management, http://cru.cahe.wsu.edu/CEPublications/eb1388/eb1388.pdf

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WSU Whatcom County Extension 1000 N. Forest St., Suite 201, Bellingham, WA 98225 (360) 778-5800 whatcom@wsu.edu