Entomopathogenic nematodes (EPN) (Heterorhabditis bacteriophora and H. megidis) and entomopathogenic fungi (EPF) (Metarhizium anisopliae and Beauveria bassiana) were evaluated for efficacy against masked chafer white grub, Cyclocephala spp., under laboratory and greenhouse conditions, as well as their efficacy against various grub stages in the field.  Under both laboratory and greenhouse conditions, additive interactions were found between EPN and EPF in their combined application against Cyclocephala spp., except a few observations that showed antagonism or synergism.  Significantly greater control occurred from the combination of a nematode and a fungus compared with a fungus alone, but not compared with a nematode alone.  The combined effect did not differ significantly for nematode and fungi applied simultaneously or at different times.  EPF had no significant impact on EPN infection and production of infective juveniles (IJs) in grub carcasses.  Nematodes alone or in combination with fungi were comparable to the insecticide Merit 75 WP (imidacloprid) against 3rd instar Cyclocephala spp in the greenhouse. Efficacy of EPF and EPN varied dramatically between field sites and conditions; EPN and EPF applied alone or in combination were less effective than Merit 75 WP in >50% field trials, but some EPN + EPF treatments were more effective than the insecticide in reducing grub numbers.  EPN and EPF showed better potential than insecticides for providing extended control of white grubs in the subsequent generation.  In addition, the sub-lethal effects of EPF on southern masked chafer, C. lurida, were investigated.  Neither M. anisopliae nor B. bassiana had a sub-lethal effect on grub weight gain, adult longevity, oviposition, pupation and eclosion.  Finally, interaction between H. bacteriophora and M. anisopliae was examined to determine the potential of the nematode in improving fungal distribution in soil.  H. bacteriophora enhanced fungal distribution in sandy loam soil without grass thatch, but not in sandy soil with thatch.  In both soil types, soil depths significantly affected nematode and fungal distribution. In water profile, M. anisopliae conidia germinated hyphae that attached to sheath of H. bacteriophora IJs, which molted to detach from the fungus.  IJs mortality and virulence were not affected by the presence of M. anisopliae. / Ph. D.

Root-feeding insects can have top-down influence on vegetative composition and ecosystem processes; however, they may respond to bottom-up factors such as soil resources, site productivity, and disturbance.  My research addressed the following questions: (1) Do disturbance (fire), vegetative composition, soil resources, and fine root standing mass influence the structure of root-feeding insect assemblages? (2) What types of roots do root-feeding insects eat, and do they forage selectively?  (3) Do root-feeding insects influence fine root productivity?  To address these questions, I studied root-feeding insect assemblages in longleaf pine wiregrass (Pinus palustris-Aristida stricta) ecosystems of southwestern Georgia, U.S.A.  On a random basis, study sites were burned at least every other year (B), or left unburned (UB) for about 9 years.  Fine root productivity and root-feeding insect abundances were sampled repeatedly across 54 random plots in UB and B sites.  In Chapter 2, I characterized spatial and temporal patterns of root-feeding insect abundance, understory plant composition, soil resource availability, and fine root standing mass within each plot.  Insect population densities were low overall, but abundance, patchiness, and diversity were greater in UB sites.  Abundance patterns were significantly related to vegetative composition.  In Chapter 3, I quantified the diet of root-feeding insects by measuring the natural abundance of carbon (C) and nitrogen (N) stable isotopes in insects and fine roots.  Using 13C abundance, I examined the contribution of warm season grass roots to insect diet, relative to the proportion of warm season grass roots within adjacent root standing crop samples; 15N abundance was used to detect omnivory.  Overall, insects appeared to be non-selective herbivores and omnivores that may alter foraging behavior to maintain a mixed diet (i.e. reducing or increasing warm season grass consumption when its abundance was high or low, respectively).  The extent of omnivory varied within and among taxa.  In Chapter 4, I estimated the top-down influence of root-feeding insects on fine root productivity by comparison of ingrowth cores with or without an insecticide treatment.  I detected a weak positive effect of herbivores on the productivity of non-grass fine roots (< 10% of fine root productivity). / Ph. D.

No description available.

Separate field studies were started in fall 2005, which continued through fall 2007, to investigate the effect of different levels of European corn borer tunneling on yield in corn grown for grain and to predict spring infestation levels of early season soil insects, specifically white grubs (Coleoptera: Scarabaeidae) and wireworms (Coleoptera: Elateridae) in cornfields. In the first study, model variables included corn growth stage and larvae per plant. In both years of this study, larvae per plant had a significant effect on grain yield. Grain yield was reduced by 13.1 and 3.65% in plants infested with four larvae per plant in 2006 and 2007, respectively. For 2006, linear regression models provided average percent yield loss per larva per plant at 4.1, 6.8, and 1.8% during late vegetative (V12), early silking (R1), and blister (R2) growth stages, respectively. Economic injury levels (EILs) were calculated based on average percent yield reductions across each growth stage and year. In the second study, no significant differences were detected in both fall and spring between two sampling methods after correcting for differences in sampling volume. Strong correlations were observed between fall and spring grub densities in both years. In 2006, fields with grub densities above the spring nominal threshold had significantly greater stand and yield in the Poncho 1250 (1.25 mg clothianidin / kernel) treatment when compared to the Poncho 250 (0.25 mg clothianidin / kernel) and untreated plots. This information was used to develop fall EILs and economic thresholds for white grubs. / Master of Science in Life Sciences

No description available.