In recent years, the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae), has become the most destructive insect of cruciferous plants throughout the world, and the annual cost for managing it is estimated to be US $I billion (168) Members of the plant family Cruciferae occur temperate and tropical climates and represent a diverse, widespread, and important plant group that includes cabbage, broccoli, cauliflower, collards, rapeseed, mustard, and Chinese cabbage, the most important vegetable crop grown in China (90), the most populous country in the world Although the diamondback moth is believed to have originated in the Mediterranean area (64), the source of some of our most important crucifers (185), diamondback moths now occur wherever cmcifers are grown, and this insect is believed to be the most universally distributed of all Lepidoptera (107) Absence of effective natural enemies, especially parasitoids, is believed to be a major cause of the diamondback moth’s pest status in most parts of the world (92) Lack of parasitoids in a particular area may have occurred because the diamondback moth is better able than its natural-enemy complex to become established in newly planted cmcifers Reports on the ability of diamondback moths to migrate long distances are numerous (19, 40, 54, 58, 108, 120,

http://web.entomology.cornell.edu/shelton/publications/pdf/Talekar-Shelton_1993_Ann_Rev_Ent_DBM.pdf

Biology, Ecology, and Management of the Diamondback Moth

Insecticides derived from the common soil bacterium Bacillus thuringiensis (Bt) are becoming increasingly important for pest management. Insecticidal crystal proteins (also called 8-endotoxins) from Bt are extremely toxic to certain pests, yet cause little or no hann to humans, most beneficial insects, and other nontarget organisms (19, 40). After proteolytic activation in the insect midgut, Bt toxins bind to the brush border membrane of the midgut epithelium and create pores that cause cells to swell and lyse (55). Technical innovations, including expression of Bt toxin genes in transgenic crop plants and transgenic bacteria, should increase the usefulness of Bt (6, 12, 35, 46, 91, 94, 109, 116). At the same time, mounting concerns about environmental hazards and widespread resistance in pest populations are reducing the value of conventional synthetic insecticides. Because Bt had been used commercially for more than two decades without reports of substantial resistance development in open field popu­ lations, some scientists had presumed that evolution of resistance was unlikely (21, 89). However, resistance to Bt was documented recently in field populations of diamondback moth in Hawaii, the continental US, and Asia (36, 42, 92a, 128, 129, 131, 138, 146, 148, 151). These reports confirmed suspicions raised by the results of laboratory selection for resistance to Bt in several major pests (102, 104, 135). Scientists in industry, government, and academia now recognize evolution of resistance to Bt in pests as the greatest threat to the continued success of Bt (18, 44, 58, 59, lOla, 108). To delay or reverse resistance to Bt in pests, we must first understand

Evolution of Resistance to Bacillus Thuringiensis

The primary role of epicuticular lipids on aerial plant surfaces is prevention of water loss, but their species-specific chemical composition and their vari­ able crystal morphology suggest other ecological functions. Among these functions is mediation of interactions between plants and insect herbivores. Epicuticular lipid extracts and individual lipid components enhance or deter oviposition, movement, and feeding. The physical structure of plant surface lipids can affect insect herbivore attachment and locomotion. Surface lipids may also affect herbivores indirectly by influencing predatory and parasitic insects. Glossy crop phenotypes are often less susceptible to insect herbivores than are normal phenotypes. The epicuticular lipids of glossy plants are usually reduced in amount and have dramatically different chemical compo­ sition and morphology. Study of the basis of the reduced suscept ibility of glossy plants can help elucidate the role of plant surface lipids in insect-plant interactions.

Effects of Plant Epicuticular Lipids on Insect Herbivores

Plant compensation for arthropod damage is a general occurrence of considerable importance in both natural and agricultural systems. In natural systems, plant species that can tolerate or compensate (e.g. recover equivalent yield or fitness) for herbivore feeding have obvious selective advantages that lead to genotype maintenance. Scientists publishing in this area often cite an optimal strategy for enhancing fitness (90, 108). In agricultural crops, reports of plant compensation mostly are concerned with yields rather than fitness (164). However, variation in compensatory response also affects sampling strategies and economic threshold levels [sensu Stern (166)] and provides viable tactic for breeding insect resistance to key arthropod pests into plants. Not surprisingly, the relative importance of the various forms of compensation in agricultural and natural systems is still relatively unknown. Therefore, a primary purpose for this review is to integrate the forms of compensation reported in the literature in the context of natural and agricultural habitats. Several previous reviews and articles have had a significant impact on the development of an understanding of plant compensatory responses. Some provide extensive lists of examples of plant compensation and the pest-yield relationship (4, 164), while others stress the complexity and interrelatedness

/pdf/plant-compensation-for-arthropod-herbivory-1kdda8vqew.pdf

Plant compensation for arthropod herbivory

1. Spiders were collected over a period of 6 months by pitfall trapping and absolute density sampling. 2. The numerical species composition of the pitfall trap catch was different to the composition of the spider community as indicated by density sampling. 3. The relationships between pitfall and density catches with time for single species were inconsistent. 4. Sex ratios differed markedly between the pitfall trap catch and those found in the density samples. 5. Possible causes of the failure of pitfall traps to reflect community composition adequately, and the implications for the use of pitfall traps in ecological sampling are discussed in the light of recent literature.

Limitations to the use of pitfall traps in ecological studies exemplified by a study of spiders in a field of winter wheat

Crop rotation is an important and effective management tactic for the Colorado potato beetle, Leptinotarsa decemlineata (Say). Colorado potato beetles in rotated potato, Solanum tuberosum L., fields tend to have lower frequencies of pesticide resistance than in nonrotated fields. One explanation that has been suggested for this phenomenon is that resistant beetles disperse less readily than susceptible beetles, and that crop rotation thereby selects for susceptibility. However, a comparison of resistance frequencies and farm practices indicates that insecticide treatment histories have a greater influence on resistance than crop rotation. Crop rotation may indirectly reduce selection for resistance by reducing the number of insecticide treatments. No evidence that crop rotation selects against resistant beetles was found.

Insecticide resistance in the Colorado potato beetle (Coleoptera: Chrysomelidae): influence of crop rotation and insecticide use.

Terminology describing crop growth stages of many commodities is often regional and can vary greatly among growers and others involved in agriculture. This lack of standardization can easily lead to misunderstandings and mistakes in crop management. To overcome this problem, the growth stages of such crops as corn, wheat, soybeans, and sugar beets have been described and given specific terms to which anyone involved in production can refer (2). Presently there is no standard terminology for describing cabbage growth stages. Although terminology such as "head formation" and "cupping" does exist, it can be ambiguous and confusing if not properly defined. More accurate descriptions and precise terminology of cabbage growth stages would be useful to those involved in cabbage production to standardize the timing of agronomic and pest control events relative to crop growth. Accurate cabbage growth stage descriptions would be particularly useful in pest management, since plant susceptibility to cabbage pests varies with the crop stage and pesticide treatments must be applied accordingly. For example, treatment thresholds for control of the major caterpillars on cabbage [diamondback moth, Plutella xylostella (L); imported cabbageworm, Pieris rapae (L); and cabbage looper, Trichoplusia ni (Hubner)] larvae have been developed according to crop growth stages (1). However, these crop growth stages must be well defined and understood by growers, researchers, extension agents, and agribusiness personnel if treatment thresholds are to be used effectively. This publication describes cabbage growth in terms of specific growth stages with appropriate terminology for each stage. Furthermore, we describe the susceptibility of these different stages to SUSCEPTIBILITY OF GROWTH STAGES TO CABBAGE PESTS

Cabbage Growth Stages

Differences in susceptibility to damage by Thrips tabaci Lindeman and the lepidopteran pest complex (Plutella xylostella (L.), Artogeia rapae (L.), and Trichoplusia ni (Hubner)) were documented and analyzed for fresh market and experimental cabbage varieties. Patterns of resistance to damage were different for Lepidoptera and T. tabaci . The commercial green varieties did not differ in susceptibility to damage by Lepidoptera but did differ in susceptibility to damage by T. tabaci . The two experimental varieties and one commercial red variety, used for comparison, were less susceptible to damage by Lepidoptera but varied in susceptibility to damage by T. tabaci . The best regression models for predicting damage rating, without including the effect of variety, included one measure of seasonal pest total and one plant parameter, head weight. Further regression analysis showed that, in most cases, the relationship of seasonal pest total to damage had the same slope for all varieties but different intercepts. This result indicates that, at a given population density, the models predict different damage ratings for the varieties, implying that nonpreference is not the only resistance mechanism operating and that mechanisms involving tolerance or antibiosis were involved. Use of regression models that test for varietal differences in the relationship of plant parameters and pest population densities to damage would be helpful in designing good assays of resistance for selection of new breeding lines.

Analysis of Resistance in Cabbage Varieties to Damage by Lepidoptera and Thysanoptera

A variable-intensity sampling method was developed to efficiently monitor all cabbage pests in commercial fields in New York while concentrating on the most important pests, Pieris rapae (L.) and Trichoplusia ni (Hubner). The field is walked in a V-shaped pattern which is divided into 10 segments. Along each segment, one to four randomly selected cabbage plants are inspected, the number depending on an estimate of population density of P. rapae and T. ni derived from previous segments. Computer simulations of this procedure were run, using random numbers from the appropriate negative binomial distribution, and the 95% confidence intervals calculated contained the true mean with frequencies between 93 and 97%. In a commercial cabbage field, a comparison of estimates of the means obtained by variable intensity sampling and fixed sample sizes of 40 and 264 plants revealed only small differences.

Variable-Intensity Sampling: A New Technique for Decision Making in Cabbage Pest Management

Insecticide usage in the Cornell University Processing (sauerkraut) Cabbage Pest Management Pilot Program was evaluated over the 4-year period from 1978 to 1981. Insecticide load was assessed in terms of dose equivalents. Insecticide efficacy was measured by percent reduction of Lepidoptera and percent reduction of plants infested with onion thrips, Thrips labaci Lindeman. Total insecticide load, in average dose equivalents per field, decreased every year from 1978 to 1980, but increased in 1981. The increase in dose equivalents in 1981 was due to multiple applications of insecticides by growers to suppress high populations of thrips. However, during the 4-year period, most insecticides were applied to control Lepidoptera, and comparing 1981 with 1978, the amount of insecticides used against Lepidoptera decreased by 49%, whereas their effectiveness increased by 54%. These results are attributed to better insecticide selection and improved timing of applications due to development of action thresholds used in conjunction with weekly field scouting. Methamidophos and the pyrethroids consistently provided the most effective control of Lepidoptera. Control of thrips by foliar insecticides was inadequate for present Food and Drug Administration standards.

C. W. Hoy

Papers

Insecticide resistance in the Colorado potato beetle (Coleoptera: Chrysomelidae): influence of crop rotation and insecticide use.

Cabbage Growth Stages

Analysis of Resistance in Cabbage Varieties to Damage by Lepidoptera and Thysanoptera

Variable-Intensity Sampling: A New Technique for Decision Making in Cabbage Pest Management

Evaluation of Insecticide Usage in the New York Processing-Cabbage Pest Management Program