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Short-rotation Crop Systems
Current Activities in the Forest Entomology Laboratory at Iowa State University
David R. Coyle and Elwood R. Hart
Introduction
Research in the Forest Entomology laboratory at Iowa State University focuses on pest-host interactions. These interactions give insight into the development of pest management programs for Populus species and hybrids in short-rotation woody crop systems. Short-rotation woody crop systems are large monocultures of fast-growing trees in plantation settings.
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Members of the genus Populus, which are easily propagated, have a rapid juvenile growth rate and desirable wood properties, are being tested widely in North America for use in short-rotation woody crop systems. The cottonwood leaf beetle, Chrysomela scripta F., (Coleoptera: Chrysomelidae) is a major defoliator of plantation Populus.
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A large proportion of young plantation Populus consists of actively-growing terminal leaves on which C. scripta immature (larval) and adult life stages prefer to feed. This damage can cause increased susceptibility to other insects and diseases, slower growth rate, deformity, or even death. The multivoltine biology of the cottonwood leaf beetle creates a potential for annual outbreaks in young plantation Populus. Forest Entomology research efforts at Iowa State University are focused on rational methods for managing the cottonwood leaf beetle in plantation Populus. By combining the use of biological control, biorational pesticides, host-plant resistance mechanisms, survey techniques, the determination of economic injury levels, and knowledge of the biology of the insect, an integrated pest management (IPM) plan is being developed to combat this pest.
Host-plant Resistance
The Populus leaf surface chemicals, alpha-tocopherylquinone and long-chain fatty alcohols, at specific ratios, act as feeding stimulants for adult C. scripta. Because individual Populus clones have varying concentrations of these leaf-surface chemicals, selecting or breeding clones without the preferred leaf surface chemical ratios or levels may be a potential way to increase host-plant resistance. A current study compares leaf surface chemical ratios before and after larval feeding to determine if this has any effect on the amounts of these feeding stimulants present.
Because of the wide range of surface chemical feeding stimulants present, the cottonwood leaf beetle may prefer certain clones. Also, CLB performance may differ on individual clones. Clones with high CLB preference and performance should be avoided in short-rotation woody crop plantations. Studies are currently underway to determine if cottonwood leaf beetle larvae show performance differences on various Populus clones with a wide range of chemical and physical characteristics.
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First-year field data show significant differences in larval performance on eight Populus clones.
Insertion of a proteinase inhibitor gene (pin2) from potato has proven an effective feeding inhibitor in laboratory studies. C. scripta larvae feeding on tissue-culture plantlets containing the pin2 gene consumed much less foliage than those feeding on plantlets without the gene. Greenhouse trials were begun in 1999 to determine if Populus trees retain this resistance in early growth stages.
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Biorational Approaches
Perhaps the most immediately promising biorational management agent for C. scripta may be the use of Bacillus thuringiensis (B.t.), a soil-borne bacterium toxic to specific groups of invertebrates. Laboratory studies at Iowa State University have evaluated the efficacy of two commercial B.t. formulations: Novodor® (Bacillus thuringiensis subsp. tenebrionis) and Raven®, (B.t. subsp. kurstaki).
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Both have induced significant differences in leaf area consumed and survival across six life stages within the B.t. treated units. Furthermore, preliminary data also suggest effective cottonwood leaf beetle control under plantation conditions using these formulations as well.
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Economic Injury Level
Because recent studies calculated beetle damage after only 2 years, it is necessary to know what effects C. scripta defoliation may have on overall biomass over the course of a short-rotation woody crop plantation rotation (£ 10 years). A study was begun at Iowa State University in 1998 to evaluate the damage and biomass loss over one complete rotation age of Populus. This current study includes Populus clones that vary in characteristics, growth, and cottonwood leaf beetle susceptibility, as some clones may show more damage early in the rotation, while others may not show damage until later.
Survey Techniques and Prediction Models
A thermal unit development model has been developed for the cottonwood leaf beetle in Wisconsin. Recent research at Iowa State University found that in central Iowa, total preimaginal development (egg to adult emergence) took 282 + 16.9 degree-days in laboratory-reared colonies, a figure very close to that calculated in Wisconsin. This laboratory value is slightly lower than that recorded in the field, and equals a 1-4 calendar day difference. Iowa State University and Mississippi State University are currently validating laboratory degree-day developmental models for their specific areas by monitoring cottonwood leaf beetle life stages in the field.
Proper scouting for the cottonwood leaf beetle is essential in order to implement an effective IPM plan. Researchers at Iowa State University and Mississippi State University also are cooperatively testing the comparative efficacy of different trap types for monitoring cottonwood leaf beetle numbers and movement. Currently, both live boll weevil and sticky traps are being used to track cottonwood leaf beetle movement across plantations and to record numbers of both cottonwood leaf beetle and ladybird beetles.
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Trends at both research sites show sticky traps to be very effective in capturing C. scripta adults. Unfortunately, sticky trap use in both areas in 1998 had to be discontinued because of a manufacturing fault in the traps. Future plans include investigating trap placement as well as trap type for monitoring cottonwood leaf beetle populations.
Conclusion
Especially over the last two decades, the biology, ecology, and impact of cottonwood leaf beetle on plantation Populus has become better understood. The ultimate goal of this research is to develop and implement an effective IPM plan for use with the cottonwood leaf beetle in plantation Populus. Short-rotation woody crop plantations already show excellent potential for use as an alternative fiber or energy source. Based upon the past, present, and planned research, adequate control of this insect should be able to increase further the productivity of these plantations for use in the 21st century.