Pest Profile: The Raspberry Crown Borer

The raspberry crown borer is a clearwing moth, similar in appearance to a yellow jacket wasp. It measures approximately 25mm in length and has a wingspan of about 30mm. The adult can be seen basking on raspberry foliage during the day.

Crown borer larvae are white with a light brown head. They possess three pairs of very small true legs and four pairs of prolegs. They range in size from a few mm to 30mm when fully grown. They are found in the raspberry crown where they Iced and tunnel into crown tissue. As they

feed, they deposit reddish brown, granular frass behind them. Their burrowing severely damages the crown and eventually affects above-ground tissue. The canes become weak and spindly and fruiting canes often collapse while the fruit is still immature. Foliage may wilt and die on affected canes. Eventually, primocanes become so few in number that the planting dies out.

Damage from this pest can often go unnoticed for some time. Symptoms are sometimes confused with winter injury, or Phy-tophthora root rot. To diagnose the problem accurately, use a spade to dig up crowns of weak plants. Shake away the soil and examine the crown for reddish-brown frass and tunnels. Use hand-pruners to cut the crowns carefully to look for crown borer larvae in and around the damaged area (Figure 3). In late fall and early spring, tug on affected canes. If crown borer is the culprit, the affected cane will usually break off at the base, revealing larvae and frass below.

Biology

The raspberry crown borer has a two-year life cycle. In late July. August and September females lay single, reddish-brown eggs on the underside of young leaves. The adult is active for about one week. Females can lay around 140 eggs in this time.

The larvae emerge about one to two months alter egg-laying and migrate down to the base of the cane. Each larva overwinters in a hibernaculum below the soil surface. In the spring, they begin boring tunnels and cavities into the crown tissue. The larvae also feed on and girdle roots and other canes belonging to the same crown. They spend their second winter within the roots of the plant. During July of the second summer, pupation occurs inside the crown and mature adult crown borers emerge between late July and September.

Management and Control

Researchers are working on the development of a pheromone lure to attract adult insects but these are not yet available.

Field sanitation, clean pruning and habitat management will help to reduce pressure from this pest. Remove nearby wild brambles to reduce the likelihood of infestations. Prune out and destroy old canes each spring, cutting them as close to the ground as possible, to destroy larvae overwintering at the base of the cane. Keep grass mowed short and weeds under control to reduce the shelter they provide to adults during the egg-laying period.

Several insecticides are registered for control of raspberry crown borer.

Diazinon 50W and Diazinon 5(H) EC: These products provide excellent control of raspberry crown borer. Spray in early spring, to control young larvae before they tunnel into the crown. Apply in a high volume of water per hectare as a drench to crowns and base of plants. Apply when new primocanes arc less than 10 cm above ground. Because this pest has a two-year life cycle, two applications, one year apart, arc necessary for control. Although many uses of diazinon are being phased out, a few limited uses, including control of raspberry crown borer, remain for now.

GuUiion 50 WSB and Sniper: These products are also very effective for crown borer larvae, but they arc very toxic to the applicator and have long re-entry periods. The spray is targeted to young larvae before they tunnel into the crown. Apply in late September or early October in a high volume of water per hectare as a drench to crowns and base of plants. These products arc being phased out and can be used on raspberries for crown borer control no later than December 31, 2012.

Altacor: This product was registered in August 2010 through the minor use registration program and will require more careful timing for control. It is effective on early instar larvae, which must ingest the product to be affected. The product must be present on foliage and eggs, to control hatching larvae in mid-late summer. We need more field experience with this product in order to develop the best strategy lor raspberry crown borer control.