Black Vine Weevil Biology and Management
Journal of the American Rhododendron Society
Richard S. Cowles
Conn. Agric. Expt. Station, Valley Lab.
P. O. Box 248
Windsor, CT 06095
Biological control of larvae
Biological control (biocontrol) is defined as suppression of a population through the action of predators, parasites, or disease organisms. There has been considerable interest in using insect pathogenic nematodes and fungal diseases against black vine weevil larvae (Zimmerman & Simons 1987, Soares et al. 1983, Simons & Van der Schaaf 1987). However, recent efforts at commercializing a fungal pathogen, Metarhizium spp., have been discontinued by some producers due to high production costs.
Unlike the more familiar plant parasitic nematodes, insect pathogenic nematodes do not have stylets, and thus are incapable of feeding on plant tissues. The commercially available species belong to two families of nematodes, Steinernematidae and Heterorhabditidae, and have similar life histories. In both families, the infective juvenile stage larva moves through moist soil, using CO2 emissions and other host associated cues to find insect larvae. Once a larva is found, the nematodes generally enter through any body opening (mouth, anus, or spiracles), and penetrate to the insect's open circulatory system. Once there, the nematode then releases symbiotic bacteria retained in a special region of their gut. In two to three days in warm soil temperatures, these bacteria multiply and produce toxins that kill the insect. The insect's cadaver becomes a bacterial soup, complete with bacterially secreted antibiotics that prevent competition with other bacteria or fungi. Over the next two weeks, the nematodes grow to adults, mate, and produce another generation. Two to three generations later (depending on the size of the grub initially infected), third stage juvenile nematodes exit from the cadaver and seek more insects. The adults of these nematodes generally are not observed unless a cadaver is broken open (Gaugler & Kaya, 1990).
The species commercially available include Steinernema carpocapsae, S. riobravis, and Heterorhabditis bacteriophora. H. bacteriophora has consistently given higher percent kill of black vine weevil larvae (Bedding & Miller 1981, Rutherford, et al. 1987, Shanks & Agudelo-Silva 1990), which may partly be due to its ability to penetrate the grub through body openings and directly through soft cuticle, using a tooth at their anterior end. This species is more difficult to grow and store, so S. carpocapsae has been more readily commercialized. A common observation has been that these nematodes can give excellent kill of black vine weevil larvae or pupae under greenhouse conditions (Evanhuis 1982; Georgis & Poinar 1984), but tend to give poor results when applied to field-grown plant material. The most likely explanations for failure of these nematodes in the field are that the soil temperatures are generally too cold for the nematodes to disperse effectively (Rutherford, et al. 1987), and that nematode antagonists may cause high mortality of these nematodes.
In spite of being flightless as adults, black vine weevils have proven to be a formidable pest with which nurserymen and rhododendron enthusiasts have had to cope. Black vine weevils' success has probably arisen from two important aspects of biology. First, the fact that larvae feed underground and the adults are nocturnal allow them to slip in and colonize, like a Trojan Horse, previously uninfested sites. Secondly, the fact that the adults can feed on such diverse and potentially toxic plants as Rhododendron and Taxus, is significant because it allows populations to develop throughout the landscape, and it also predisposes populations of this organism to develop more finely tuned metabolic machinery to detoxify man-made insecticides.
The challenge in integrated pest management is to turn a pest's traits to our advantage. The fact that adults are flightless means that we have an opportunity to exclude them from feeding on plants we wish to protect. Unclimbable barriers are probably the most underutilized, most common-sense tool to combat root weevils. Since the adults hide during the day, we can respond by using trunk wraps for monitoring populations and for mass trapping. Finally, since adult weevils have strong feeding preferences, we may be able to devise baits for more efficiently applying insecticides.
For large scale nurseries, it is important to realize that adults may be emerging and be ready to lay eggs at various times of the year. Overwintering prepupae will develop to adults in late winter if held within containers in warm hoophouses. The next flush of adults may be the overwintering two-year old beetles, and the last group to emerge would develop from prepupae overwintering in cold field soils. This complexity, and the inability to obtain adequate control of adults with currently registered insecticides, has led to chronic, low level infestations in many susceptible plant materials. Only by combining a fresh approach of maintaining growing areas free of adult weevils (using exclusion), by monitoring adult populations (to detect failure in exclusion), and more effective use of insecticides can black vine weevil be eliminated from the nursery trade.
|Introduction & General Biology||Management Options & Adulticides||Adult exclusion & Conventional larvicides|
|Biological control of larvae & Summary||References|
|YEYS IN CONNECTICUT ARTICLE|