Effect of gamma irradiation of gravid Tetranychus desertorum, T. urticae and Oligonychus ilicis (Trombidiformes: Tetranychidae) females on the viabilities and durations of F1 life stages

Abstract

The objective of the study was to evaluate—as an alternative phytosanitary treatment—the effects of gamma irradiation of gravid spider mite females on the viabilities and durations of F1 eggs and deutonymphs. The study focused on Tetranychus urticae Koch, Tetranychus desertorum Banks and Oligonychus ilicis (McGregor) (Trombidiformes: Tetranychidae). Each mite species was identified under a stereomicroscope, and individual female mites of each species were each taken with a fine-tipped brush and transferred to a Petri dish—i.e., 1 female mite individualized per dish with 32 dishes per species. Each dish with a Tetranychus spp. female was provided with a jack bean (Canavalia ensiformis [L.] DC) (Fabales: Fabaceae) leaf, and each dish with an O. ilicis female was provided with a coffee (Coffea arabica L.; Gentianales: Rubiaceae) leaf. Gravid female mites were irradiated in a Gammacell 220 cobalt-60 source—dose rate 0.486 kGy/h—with doses of 0 (control), 100, 200, 300, and 400 Gy with 32 replications per mite species. Evaluations were made every 24 h for 22 d after irradiation by counting the numbers of eggs, deutonymphs, and adults and by recording the mortality in each stage. Statistical analyses were done with generalized linear models with either the quasi-binomial distribution or the Gaussian distribution—as appropriate—for analyses of mortality rates and durations (d) of the F1 eggs and deutonymphs of the 3 mite species. A 3 × 5 factorial experimental design was used in which factor A (3 levels) pertained to the species and factor B (5 levels) pertained to the 4 radiation doses plus the control. For Factor B polynomial regressions of dose rates on biological effects were used. The results showed progressive decreases in egg and deutonymph viabilities that were proportional with progressive increases in radiation doses. Against O. ilicis 200 Gy prevented all F1 neonates from developing beyond the first instar. Against T. urticae and T. desertorum 300 Gy prevented all F1 neonates from developing beyond the first instar. Irradiation of gravid females caused the durations of the F1 egg stage and the F1 deutonymph stage to be prolonged.