Resource utilization and larval survival of the bud borer, Crocidosema aporema (Lepidoptera: Tortricidae), in soybean (Glycine max)

Abstract

Crocidosema aporema is the most important soybean pest in Argentina. Larvae feed on vegetative and floral buds, and since more than one larva is usually found in a bud, the number of larvae per bud can affect available resources. The objective of this work was to evaluate larval performance according to the larval density per bud, the development strategy of a larvae that arrives to an already occupied bud, and adult ovipositional strategy. We analyzed the maximum densities of eggs and larvae, their spatial distribution, the amount of resources needed for larvae development, and the effect of increasing larval density on dispersal and survivorship. Soybean during V₅ phenological stage showed 76.7% of the plants with eggs, and 27% with larvae. Soybean during V₉ stage (2.5 eggs per plant and 5 buds per plant) had 74.3% of the plants with eggs, and 46% of the plants with larvae. We found an aggregate spatial distribution in all cases. Results suggest that adults avoid high densities of eggs or larvae per bud during the oviposition. Consumption during the last two larval instars represents 95.7% of total larval consumption. We infested soybean plants with one to four first instar larvae per bud (T₁, T₂, T₃, and T₄ treatments, respectively) in a field experiment. Larval survival showed significant differences between T₁ and T₄. Initial density affected the proportion of larvae that left the bud in the fourth instar looking for other buds. Mean maximum distance run by larvae increased with the initial number of larvae per bud. The number of consumed organs per plant also increased with a higher number of larvae per bud; however, the ratio between mean total of consumed organs and mean surviving larvae was not significantly different from the mean. A first instar larva that colonizes an occupied bud can either remain or search for a new one. If it remains, survival will change with number of larvae; if it searches for another unoccupied bud, survival will be the product between survival of T₁ and the probability of colonize it, which means a lower value. These values would be even lower in the field, due to the occurrence of diverse predators, therefore being advantageous for a first instar larva to remain in the same bud, even if it is already occupied.