Parasitism by the endoparasitoid, Cotesia flavipes induces cellular immunosuppression and enhances susceptibility of the sugar cane borer, Diatraea saccharalis to Bacillus thuringiensis

Abstract

Cotesia flavipes Cameron (Hymenoptera: Braconidae), is a gregarious larval endoparasitoid of the sugarcane borer, Diatraea saccharalis Fabricius (Lepidoptera: Crambidae). The aim of this research was to analyze cellular immunosuppression of D. saccharalis parasitized by C. flavipes in terms of encapsulation, melanization, and hemocyte nodule formation. The encapsulation assay was done 1 and 6 days after parasitoid oviposition. In addition, the susceptibility of parasitized and nonparasitzed larvae to Bacillus thuringiensis HD 73 strain was assessed. 3, 12, and 24 h after bead injection; the percentages of encapsulation were significantly higher in unparasitized larvae compared to larvae parasitized 1 and 6 days after oviposition. Interestingly, there was a significant reduction in numbers of beads encapsulated at 1 day after oviposition compared to 6 days, and unparasitized larvae. The percentage of melanized beads decreased significantly in parasitized larvae compared to control. There was a reduction in the number of nodules in parasitized larvae compared to unparasitized controls. Larvae that were injected with polyndavirus 24 h before beads were injected showed significantly reduced encapsulation responses relative to control larvae. The D. saccharalis parasitized by C. flavipes exhibited higher susceptibility to B. thuringiensis. These results suggest that parasitization induced host immunosuppression, and the immunosuppression factors could impair the defense capacity against microbial pathogens--causing an increase in pathogen susceptibility.

Figure 1.

Photomicrographs of Sephadex A-25 beads after recovery from Diatraea saccharalis larvae showing the scoring system for encapsulation: (A) encapsulated bead; (B) bead with different adherent cells; (C) melanized beads; (D) bead with no adherent cells. High quality figures are available online.

Figure 2.

Effects of parasitization on encapsulation responses of Diatraea saccharalis larvae to Sephadex A-25 beads. Each larva received 10–15 beads, and 10 beads were recovered and scored for encapsulation. Graph bars represent mean percentages (of n larvae) ± standard error. ppo = post-parasitoid oviposition (A). Beads were recovered (by dissection) from larvae after 3 h. (B) Beads were recovered after 12 h. (C) Beads were recovered after 24 h. High quality figures are available online.

Figure 3.

Effects of parasitization on capsule thickness (A) and melanization (B) of Sephadex A-25 beads recovered after incubation (3, 12, or 24 h) in the hemocoel of fifth-instar Diatraea saccharalis larvae. (A) Graph bars represent mean thickness ± standard error of capsules surrounding n beads from n larvae. (B) Graph bars represent mean percentage (± standard error) of beads (from n larvae) showing associated melanization. High quality figures are available online.

Figure 4.

Effects of filter-purified polydnavirus on encapsulation responses of fifth-instar Diatraea saccharalis larvae to Sephadex A-25 beads. Larvae received I wasp equivalent (eq.) of filtered calyx fluid. Twenty-four hours later, each larva received 10–15 beads, which were recovered after a 24 h incubation. Graph bars represent mean percentages (of n larvae) ± standard error. High quality figures are available online.

Figure 5.

Effect of Cotesia flavipes parasitization on hemocyte nodule formation of Diatraea saccharalis in response to nonpathogenic bacteria, E. coli, infection (5×10⁶ cells/larva). Each measurement was replicated 10 times. ‘NP’ and ‘P’ represent nonparasitized and parasitized larvae, respectively. Error bars represent standard deviations. Different letters above error bars indicate significant difference at a = 0.05 (LSD test). High quality figures are available online.

Figure 6.

Parasitism by Cotesia flavipes enhances pathogen susceptibility of Diatraea saccharalis. Synergistic pathogenicity between Bacillus thuringiensis HD73 strain (500 Mg/ml diet) and the parasitism. Each concentration was replicated three times with 10 larvae per replication. Error bars represent standard deviations. Different letters above indicate significant different among means at Type I error = 0.05 (LSD test). High quality figures are available online.