Phenology, Natural Enemies, and Management of Lepidopteran Pests of Cole Crops in Alabama

Abstract

This thesis first looks at the seasonal abundance of the three key pests of cole crops, Plutella xylostella (L.) (Lepidoptera: Plutellidae), diamondback moth; Trichoplusia ni (Hübner) (Lepidoptera: Plutellidae), cabbage looper; and Pieris rapae (L.), imported cabbageworm in central Alabama on cabbage and collards. It also does a survey of the parasitoid complex of these pests. The only parasitoid recovered from P. xylostella was an ichneumonid wasp, Diadegma insular (Cresson) reaching parasitism rates of 57%. Very sporadic populations of T. ni resulted in very few parasitoids being recovered. Lastly, the dominant parasitoid of P. rapae was the pteromalid wasp, Pteromalus puparum (L.) which reached parasitism rates of 42.5%. The second objective of this thesis is to evaluate two species-specific pheromone-based experimental attracticide formulations against these pests: Last Call TM DBM for P. xylostella and Last Call TM CL for T. ni. Laboratory toxicity experiments were first used to confirm the effectiveness in killing conspecific males. In replicated small plots of cabbage and collards, an attracticide treatment receiving the two Last Call TM formulations, each applied multiple times at the rate of 1600 droplets per acre was compared against Bacillus thuringiensis subspecies kursatki (Bt) spray at action threshold and a negative untreated control. Efficacy was measured by comparing among the three treatments the following parameters: male capture in pheromone-baited traps, larval counts in plots, and crop damage rating at harvest. The results show that LastCallTM provided acceptable pest control comparable to Bt in three of the four seasons. Efficacy of LastCallTM was shown to be dependent upon lepidopteran population densities, which fluctuated from season to season. In general, LastCallTM was effective at low to moderate population densities of the three species, such as typically occurs in the fall, but not in the spring when high P. rapae population pressure typically occurs in central Alabama. In no case did we record significant trap shutdown in LastCallTM plots suggesting that elimination of males by the toxicant (permethrin), rather than interruption of sexual communication, was probably the main mechanism behind the success of the attracticide formulations. The third objective was to evaluate several reduced-risk insecticides for the management of these pests. The following formulated sprays were evaluated: Dipel® (Bacillus thuringiensis subspecies kursatki), XenTari® (B. thuringiensis subspecies aizawai), Dipel+XenTari (a premixed test formulation consisting of both subspecies of B. thuringiensis), Entrust® (a formulation of spinosad), and Novaluron (insect growth regulator). An action threshold of 0.5 cabbage looper equivalents (CLE) per plant was used to determine the need for insecticide applications. Insecticide efficacy was determined by comparing densities of larvae and immatures (larvae + pupae) of each pest species, crop damage ratings, densities of key non-target arthropods, and number of insecticide applications in plots treated with each material versus untreated control plots. All five reduced-risk insecticide formulations were effective in reducing infestations of the three pests and in providing marketable cabbage and collards in Alabama. No significant effects of insecticide treatments were recorded in the numbers of spiders and lady beetles found per plant. The results also suggest that the 0.5 CLE action threshold can be used to produce marketable cabbage and collards with only minimal applications of reduced-risk insecticides. The main goal of this project was to evaluate some componanents of an integrated pest management program to control these pests. Hopefully this information can be used to help growers in Alabama build a successful integrated management program for future use.