A case of plasticity in the symbiotic associations of an ambrosia beetle

Avocado, Persea americana Mill., is an important fruit crop that is currently threatened by a mortal disease known as Laurel Wilt (LW). The disease is caused by the fungal pathogen, Raffaelea lauricola, and transmitted by the redbay ambrosia beetle, Xyleborus glabratus. Recently, R. lauricola was found associated with other resident ambrosia beetles in south Florida avocado orchards. One species, Xyleborus bispinatus, is frequently found breeding in LW-affected avocado trees and was demonstrated capable of transmitting R. lauricola to avocado trees under greenhouse conditions. Previous studies suggest that, unlike other ambrosia beetles, X. bispinatus does not have a clear primary fungal symbiont. Controlled symbiont experiments were used to evaluate the fitness of X. bispinatus feeding exclusively on R. lauricola or other fungi through several generations. Overall, X. bispinatus developed and reproduced feeding on four different fungi, including three Raffaelea species. The number of offspring were similar when X. bispinatus was fed individually on different Raffaelea spp. but significantly greater when offered several symbionts simultaneously (i.e., control). Surprisingly, X. bispinatus showed the highest fitness when fed on a fungus never reported in association with this beetle. This study provides relevant information for future research on the interactions between plant pathogenic and non-pathogenic symbionts, symbiont displacement, larval feeding habits, and the nutritional value of different fungi for ambrosia beetles.