Utah Pests News Fall 2009

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Raspberry Horntails Severe in 2009

Raspberry is an important agricultural crop in Utah.  Utah growing conditions are conducive to caneberry production, and there is a strong local market for the tasty fruits.  During 2009, infestation of raspberry canes by the cane boring insect, raspberry horntail (Cephidae: Hartigia cressonii), has been severe.  A Bear Lake grower reported more than 80% infestation of primocanes in one raspberry variety, ‘Killarney’.  Horntails attack the vegetative, or primocanes, of both annual- and ever-bearing varieties.  Some basic life history information on raspberry horntail is available in the literature (Perry et al. 2003), but details on its biology and pest management are lacking.  Research was initiated in 2009 to gain a better understanding of the insect’s life history in raspberry in northern Utah.  In upcoming years, research will focus on raspberry varietal susceptibility, developing a degree day model to assist with control timing, and testing insecticidal and biological control products.

The raspberry horntail is a type of wood wasp.  It is named for the small single spine or horn on the tip of the tail (Fig. 1).  It was first documented in Utah in the 1980s, and has since displaced the rose stem girdler, a flatheaded beetle, as the most common borer in the upper portions of canes.  A third species of caneborer found in Utah is the raspberry crown borer, a clearwing moth, that attacks the base of canes and is often found co-existing with horntail in the same plantings.

 
  Figure 1. The larva of the raspberry horntail has a spine (see arrow), giving the insect its name.

Observations in 2009 suggest there is a single generation of horntail in northern Utah.  Larval development was spread out suggesting that adults are active and lay eggs over an extended period of time.  Adults emerge from overwintering sites within last year’s canes beginning in the early spring, and females insert eggs under the bark of the current year’s primocanes.  Eggs are difficult to find, but once young larvae hatch, they tunnel upwards inside the cane.  We observed narrow trails starting low on the canes.  Egg-laying is initiated in the early spring, so eggs are likely laid into new cane growth soon after it emerges from plant crowns.  The trails of small larvae spiral within the canes, and were associated with the ring of cambial tissue under the bark layer.  This suggests that young larvae preferentially feed in the nutrient-rich tissues of the cambium.  With careful observation, dark trails of small larvae could be seen under the bark of canes.

More obvious symptoms of cane infestation were not observed until the larvae were larger and had tunneled to the tops of canes.  After substantial larval feeding, cane tips wilted (Fig. 2) and leaves turned brown and dried up (Fig. 3).  Older larvae spent at least a week tunneling throughout the terminal portion of the canes causing dieback.

 
 
  Figures 2 and 3. Wilting (top) and drying leaves (bottom) caused by feeding of the horntail larva inside the cane.


At least two types of parasitoid wasps were observed attacking horntail larvae within the cane tips.  One is an ectoparasite (feeds externally on its host) where multiple parasitoid larvae were observed feeding on a single horntail larva (Fig. 4).  Multiple pupae of the ectoparasite were found along with the dead and shriveled horntail larva within a cane (Fig. 5).  A second type of parasitoid is an endoparasite (develops within its host).  Based on appearance of the adults, the endoparasite may be an ichneumonid wasp (Fig. 6).  Adult parasitoids will be collected as they emerge from canes for positive identification.

 
 
 
  Figures 4, 5, and 6. While examining horntail larvae, we found ectoparasites (top), ectoparasite pupae (middle), and endoparasites (bottom), possibly an ichneumonid wasp.

Once horntail larvae are mature, they turn around and burrow downwards through the cane pith to form a cavity in the lower part of the cane in which to spend the winter.  The first horntail pupation chambers were observed within canes in late July in northern Utah.

We have learned some important points about raspberry horntail biology: egg-laying appears to begin in the early spring just after new canes emerge from the crowns of plants, although there appears to be a single generation, larval activity occurs from May to at least August, wilting of the cane tip isn’t observed until horntail larvae are older and have fed substantially in the upper cane, several types of parasitoids attack the horntail in northern Utah, and horntails pupate in the lower parts of canes where they will be more protected from winter conditions.

Using this information on horntail biology to improve their management, we can recommend: 1) begin insecticide treatments just after new canes begin to grow in the spring (avoid insecticide applications during bloom that will harm pollinators), 2) look for early symptoms of young horntail larvae within canes – dark trails just under the bark, 3) prune out infested cane tips when wilting symptoms are first noticed, 4) protect parasitoids that provide natural biological control of horntail larvae by avoiding toxic insecticides once horntails are feeding in cane tips, and 5) prune canes in the late winter to early spring before horntail adults emerge – canes of ever-bearing varieties can be completely pruned back, and floracanes of summer-bearing varieties that appear unhealthy may contain horntail pupae and should be removed.



-Diane Alston, Extension Entomologist


Reference

Perry, E. J., M. P. Bolda, and L. J. Bettiga. 2003. Caneberries: Raspberry horntail. UC Pest Management Guidelines. UC Agriculture and Natural Resources Publication 3437 (2 pp.). University of California, Davis, CA.