Effects of microsporidiosis and food availability on the two-spotted lady beetle, Adalia bipunctata L., and convergent lady beetle, Hippodamia convergens Guérin-Méneville.

Abstract

Two species of microsporidia have been described from lady beetles that are commercially available for biological control: Nosema adaliae from the two-spotted lady beetle, Adalia bipunctata L., and Tubulinosema hippodamiae from the convergent lady beetle, Hippodamia convergens Guérin-Méneville. These pathogens delay larval development under controlled conditions, but little is known regarding the effects that microsporidia cause when their hosts are subjected to stressful conditions that are often experienced in nature. In this study, the combined effects of microsporidiosis (N. adaliae on A. bipunctata and T. hippodamiae on H. convergens) and irregular food availability were observed on host fitness (larval development and mortality, sex ratios, and adult morphometrics). For each beetle species, 24 h-old larvae were provided either an uninfected or microsporidia-infected conspecific egg. After the egg was eaten, some larvae were provided an abundance of aphids daily, whereas others were provided aphids on an irregular basis. Development was delayed significantly for larvae that consumed a microsporidia-infected egg, and for those fed irregularly. For A. bipunctata, a significant interaction was observed between infection status and food availability. This suggests that N. adaliae-infected A. bipunctata larvae that have an irregular supply of aphids undergo further developmental delays than those with a generous food supply. This interaction was not observed for T. hippodamiae-infected H. convergens. For both species, larval mortality and sex ratios did not differ significantly, regardless of infection status or food availability. Adults that were fed daily as larvae were significantly larger than those fed irregularly. However, the elytra of N. adaliae-infected A. bipunctata were significantly larger than the elytra of their uninfected cohorts, and T. hippodamiae-infected H. convergens had wider pronota and head capsules than uninfected H. convergens. Because N. adaliae and T. hippodamiae prolong larval development of their respective hosts under controlled conditions, one would expect these pathogens to cause more profound effects when their hosts experience stressful conditions. The results from this study indicate that this was the case for A. bipunctata, but not for H. convergens.