Effects of Plant Growth-Promoting Rhizobacteria (PGPR) Treatment of Cotton on the Oviposition Behavior of Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae)

Abstract

Treating crops with a group of naturally-occurring root bacteria, termed plant growth-promoting rhizobacteria (PGPR), has been shown to increase plant growth and enhance plant health in different ways. Although much is known about the effects of PGPR treatment on plant growth and disease resistance, very few studies have explored how the use of PGPR may affect plant-insect interactions. This study was carried out to investigate the effect of PGPR treatment of cotton plants on the host location and acceptance behavior of a generalist herbivorous insect, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) (beet armyworm). Specifically, the goal of this study was to investigate the effects of PGPR treatment of cotton plants on the oviposition behavior of S. exigua with experimental design considerations for the presence of headspace volatile organic compounds which may affect this behavior. In chapter II, choice and no-choice oviposition studies were used to evaluate the influence of treating cotton plants with PGPR on the oviposition behavior of S. exigua. In choice and no-choice bioassays, the oviposition behavior and preference of females was compared among four treatments: i) untreated cotton plants (control), cotton plants treated with ii) Bacillus pumilis strain INR-7 (a single PGPR strain), iii) Blend 8 (a blend of 4 PGPR strains), and iv) Blend 9 (a blend of 4 PGPR strains). In choice oviposition bioassays, females preferred to oviposit on untreated cotton plants (control) compared to any of the three PGPR treatments. In particular, Blend 9-treated plants had significantly fewer eggs compared to untreated plants. In no-choice bioassays, however, similar numbers of eggs were deposited on untreated plants and PGPR-treated plants. These results show that S. exigua females are capable of distinguishing between the PGPR-treated and untreated plants, as well as between some of the PGPR-treated plants. More notably the results showed that PGPR treatments influence the oviposition behavior of S. exigua and that some PGPR treatments may be able to increase plant health by reducing oviposition by lepidopteran pest species. In chapter III, three different designs of no-choice oviposition studies used to evaluate the oviposition behavior of a S. exigua were compared and discussed. Females were allowed to oviposit overnight on one of four treatments: i) untreated cotton plants (control), cotton plants treated with ii) Bacillus pumilis strain INR-7 (a single PGPR strain), iii) Blend 8 (a blend of 4 PGPR strains), and iv) Blend 9 (a blend of 4 PGPR strains). In two of the designs, all four of these treatments were evaluated, but distances separating individual cages were increased in the second design because it was suspected that there was contamination between treatments from volatile organic compounds (VOCs) released by the plants. Reductions in P-values with increased distances between individual cages correspond with the hypothesis that VOCs associated with the headspace of different treatments studied are capable of diffusing through cloth and affecting the oviposition behavior of S. exigua. This hypothesis was further supported by a third design evaluating only two of the treatments (the untreated control and Blend 9), which reduced the numbers of treatments whose VOC profiles may be design contaminants. A comparison of the results reveals the potential for PGPR treatment of plants to either stimulate or reduce egg-laying behavior of Lepidoptera under short-range, no-choice conditions.