Effects of sweet fennel, Foeniculum vulgare var. dulce, essential oil on the microsporidian pathogen Vairimorpha adaliae and its host, the two-spotted lady beetle, Adalia bipunctata L. (Coleoptera: Coccinellidae)

Abstract

The two-spotted lady beetle, <i>Adalia bipunctata</i> L., is commercially available for biological pest control in Europe and North America. It is susceptible to the microsporidium <i>Vairimorpha adaliae</i>, a spore-forming fungal pathogen which prolongs larval development. Sweet fennel essential oil, <i>Foeniculum vulgare</i> var. <i>dulce</i>, is known to have antifungal properties and has been used for the management of both the microsporidium <i>Nosema ceranae</i> in honeybees and the late blight pathogen, <i>Phytophthora infestans</i>, in tomato and potato plants. However, sweet fennel essential oil has not been investigated for the management of <i>V. adaliae</i> in <i>A. bipunctata</i>. In this study, the effects of sweet fennel essential oil on larval development time, larval mortality rate, adult sex ratio, and microsporidian infection (spore counts) in <i>V. adaliae</i>-infected <i>A. bipunctata</i> were examined. Development time, mortality rate, and sex ratio of larvae fed green peach aphids, <i>Myzus persicae</i>, and water did not differ significantly from larvae fed aphids and 10%, 20%, or 30% fennel, respectively. Although spore loads did not differ significantly, heavier infections were observed in increased fennel concentrations with an increase of 12.83%, 61.33%, and 58.24% in the 10%, 20%, and 30%, respectively. This study suggests that fennel essential oil is safe to use on <i>A. bipunctata</i> and may provide additional energy to the host beetle to withstand the effects of the microsporidian pathogen as larval development time was unaffected, despite heavier infections. Understanding the role of plant secondary compounds on pathogens in lady beetles will help to ensure that beneficial insects remain effective biological control predators as an alternative to synthesized pesticides.