Selectivity of water-based extracts of Serjania spp. on Tetrastichus howardi (Hymenoptera: Eulophidae), an endoparasitoid of Plutella xylostella (Lepidoptera: Plutellidae)

The use of botanical insecticides is a sustainable alternative for the regulation of Plutella xylostella (Linnaeus, 1758) (Lepidoptera: Plutellidae) in brassica crops. Similarly, biological control with parasitoids can be an effective tool when integrated with bioinsecticides. However, the safety of natural insect enemies must be prioritized within pest control programs. The objective was to evaluate the effects of the water-based extracts of Serjania erecta Radlk (Sapindaceae) and Serjania marginata Casar (Sapindaceae), at 5% and 10% concentrations, on the reproduction of Tetrastichus howardi (Olliff, 1893) (Hymenoptera: Eulophidae) in larvae of P. xylostella, topically treated with extracts. In addition, we evaluated the survival of adult females of T. howardi exposed to the respective extracts of Serjania spp. For this purpose, two bioassays were conducted. In the first, 4th instar larvae of P. xylostella were selected, and 20 μL of each treatment was applied topically (S. erecta 5%, S. erecta 10%, S. marginata 5%, S. marginata 10%, distilled water) in each larva and then exposed to T. howardi parasitism for 48 hours. The percentage of parasitism and emergence (%), life cycle length (days), total number of progeny, sex ratio and adult longevity were evaluated. In the second bioassay, the ASPECLE (Evaluation of Pesticide Selectivity under Extended Laboratory Conditions) system, adapted from the International Organization for Biological Control (IOBC) standard model, was used. In this bioassay, five cabbage leaf disks were introduced in each glass cylinder of the ASPECLE system, duly treated with distilled water, aqueous extracts of S. erecta and S. marginata, at concentrations of 5% and 10%, or acephate. Subsequently, ten adult females of T. howardi were released in each cylinder and the system was activated. Parasitoid survival was evaluated for 24, 48, 72, 96 and 120 hours. The aqueous extracts of S. marginata and S. erecta at concentrations of 5% and 10% did not interfere in the parasitism of T. howardi on 4th instar larvae of P. xylostella. Field experiments should be developed to prove the negative effect of aqueous extracts of S. marginata and S. erecta on other biological characteristics of T. howardi. As a precaution, releases of T. howardi and sprays of aqueous extracts of S. marginata and S. erecta should be made at different times for the biological management of P. xylostella.