ABSTRACT: The majority of ectotherms exhibit a pattern of decreasing size with increasing temperature, known as the temperature-size rule (TSR). Despite the extent of this pattern, the effect of seasonality on the TSR is poorly understood. We use Helicoverpa zea (corn earworm) to determine the effects of seasonal dormancy programming (pupal diapause) on the TSR, lipid storage, and larval development. H. zea did not conform to the TSR over the seasonally-relevant temperature range where pupae entered diapause. H. zea increased in both lipid and lean mass with increasing temperature for 16°C to 20°C in both diapause-destined and non-diapause treatments. Diapausing H. zea were fatter across temperatures without having more lean mass, and the slope of the relationship between lean mass and temperature was not different between diapausing and non-diapausing pupae. Across temperatures, diapause-destined larvae increased their consumption but not their digestive efficiency, supporting increased pupal lipid mass. We found threshold weights for entrance to and exit from diapause, and diapause-destined larvae altered the timing for pupation, further reinforcing the importance of size to seasonal diapause. In conclusion, predictions of seasonal effects on body size must incorporate cues other than temperature, to avoid inaccurate or incomplete models of development and body size.
The commercialization of cotton and corn, genetically engineered to produce a caterpillar-specific insecticidal protein (Bt toxin) has focused renewed attention on the issue of southward migration of Helicoverpa zea because it could have a major effect on the rate at which this pest evolves resistance to the Bt toxin. Corn is a major host of H. zea and since there is presently limited use of transgenic Bt corn in the Northern corn-belt southern migration from this corn could result in a larger overall refuge for susceptible H. zea and that would decrease the rate of resistance evolution We applied a stable carbon isotope technique to test the hypothesis that H. zea migrates North in early summer, feeds on corn, then migrates back South to feed on cotton. We analyzed the relative abundance of the naturally occurring isotope, 13C, incorporated in wing cuticle of a moth as an indicator of the photosynthetic pathway (corn=C4, cotton=C3) of the plant that had been fed upon by it when it was a larva. Our carbon isotope analyses of H. zea, captured in pheromone traps, in the Brazos River Valley, Texas, demonstrated that at any given date from late summer to early fall in 2000, 40-100 percent of the moths developed on C4 host plants and were present as adults in a cotton growing area where there were no known C4 hosts suitable for development. Our investigation has also shown significant morphometric differences in moths from C3 and C4 hosts. Moths that fed upon a C4 host during the larval stage typically had larger wing length and were heavier than moths that fed upon a C3 host. Host plant quality during larval development may affect fluctuating asymmetry in adult characteristics, such as wing length, causing greater asymmetry in moths whose larvae fed on sub-optimal hosts. Apparently, there was no effect of host plant type on the degree of wing length asymmetry measured in the Texas moth samples, however, there was a significant effect of DATE, and an interaction effect o.
Helicoverpa zea (Boddie), the corn earworm, is a polyphagous caterpillar pest found throughout the United States and is a key pest of sweet corn. Chapter one is a review of literature relevant to the biology, ecology and management of H. zea in United States sweet corn production. Chapter two evaluates the predictive ability of male moth pheromone trap catch alone compared to a model that incorporates multiple factors on the biology and development of H. zea and the environment. Chapter three tests the efficacy of insecticides, registered for use against H. zea in sweet corn, in context to important timing windows during sweet corn ear development, as outlined in chapter two. The epilogue summarizes conclusions and identifies areas of future research. Chapter one is a comprehensive review of the literature relating to the biology, ecology and management of H. zea in United States sweet corn production. First, H. zea behavior, development, host interactions and ecology are reviewed, including host range, dispersal and migration, diapause and overwintering. Next, integrated pest management (IPM) practices for control of H. zea are discussed. Then, current tools including cultural, biological, chemical and transgenic controls for H. zea are reviewed. Finally, research needs likely to be of importance for management of H. zea in coming years are outlined. Female H. zea oviposit on sweet corn silks and yield loss occurs when neonates migrate into the ear under the husk to feed because a single larva can cause complete economic loss if the ear is for fresh market purposes. Chapter two examines current integrated pest management (IPM) guidelines for sweet corn that use pheromone trap-captured male H. zea moths to inform management decisions compared to models inclusive of additional factors relevant to ovipositional behavior or development of H. zea. Results of logistic regression and predictive discriminant analyses demonstrate that using multiple environmental and biological factors do, in fact, provide a higher predictive power than pheromone trap catch alone. These results show that IPM strategies to control H. zea damage in sweet corn should use multiple biological and environmental factors important for oviposition and infestation, and that pheromone trap catch alone is not the best predictor of damage at harvest. Chapter three draws on the conclusions of chapter 2, asking whether sweet corn can be protected more effectively if insecticides are applied to target the most attractive silking periods for female H. zea oviposition. The relationship between insecticide application timing from tassel through silk stages and marketable yield at harvest were evaluated in the field. Results were compared to yields resulting from current IPM recommendations for the northeast United States. The effectiveness of three registered insecticides (methomyl, chlorantraniliprole and lambda-cyhalothrin), each representing a different class of insecticide, were evaluated. Significant yield differences among insecticides and timing treatments were detected and the combined effects of active ingredient with timing determined the extent of H. zea damage. The efficacy of chlorantraniliprole as an effective means of H. zea control in sweet corn was unclear. In year 1, there was no significant effect of insecticide type or application timing, but there was a significant interaction effect between factors. In year 2, there was a significant effect of insecticide type. Chlorantraniliprole treatments resulted in significantly higher percentages of sweet corn ears compared to lambda-cyhalothrin. There was also a main effect of application timing. Four insecticide applications made from 50% tassel to 25% dry silk resulted in significantly higher percentages of clean ears at harvest compared with a single insecticide application made at 50% tassel. Compared to other timing treatments, however, there were no significant differences. 4 The epilogue provides a summary of conclusions reached from chapters one through three. This section also discusses areas of future research that include plant-insect dynamics, chemical ecology and possibilities for advancement of IPM strategies for H. zea management in the 21st century. 5.
