Virulence, horizontal transmission, and sublethal reproductive effects of Metarhizium anisopliae (Anamorphic fungi) on the German cockroach (Blattodea: Blattellidae)

Abstract

Virulence of Metarhizium anisopliae (Metschnikoff) Sorokin strain EAMa 01/121-Su against the German Cockroach, Blatella germanica (L.), was determined using four concentrations ranging from 4.2 × 10 6 to 4.2 × 10 9 spores per milliliter. The LD 50 value was 1.4 × 10 7 spores per milliliter (56,000 spores per cockroach) and LT 50 values were 14.8 days and 5.3 days for 4.2 × 10 8 and 4.2 × 10 9 spores per milliliter, respectively. An experiment was conducted to evaluate whether a fungal transmission could exist among infected and healthy cockroaches. Percentage mortality at a ratio of 1:10 of infected to unexposed cockroaches was 87.5% and LT 50 was 12.2 days, which indicated the potential of this strain to be horizontally transmitted and to rapidly spread the infection in the insect population. The effect of a sublethal dose (ca. LD 60) of M. anisopliae EAMa 01/121-Su strain, applied topically on German cockroaches, was studied by reciprocal crossing. Othecal production, oothecal hatchability, and nymphal production declined upon exposure to M. anisopliae EAMa 01/121-Su strain. The mean number of oothecae laid by female was progressively and significantly reduced by fungal treatment from second oviposition period onwards. Oothecal hatch of fungally challenged females was reduced by 46–49%, oothecal viability by 48–85%, and nymphal production by 22–35%. Only treated females showed an effect on oothecal production, oothecal hatch, and nymphal production, although oothecal hatch was also governed by treated males at a higher significance level. Our results on virulence and horizontal transmission of fungal conidia of M. anisopliae EAMa 01/121-Su strain and its sublethal reproductive effects on German cockroach females are discussed in terms of its potential to decrease the pest status of B. germanica in the short and long terms.