Obligate feed requirement of Fusarium sp. nov., an avocado wilting agent, by the ambrosia beetle Euwallacea aff. fornicata

The ambrosia beetle, Euwallacea aff. fornicata Eichhoff was first recorded in Israel in 2009. The symbiotic fungus Fusarium sp. nov., carried in the mandibular mycangia of the beetle, is responsible for typical wilt and dieback symptoms inflicted on avocado (Persea americana Miller) trees. The beetle-fungus complex has become a serious threat to the future of the avocado industry in Israel and elsewhere. When reared on Petri dishes, inoculated with 7-day-old cultures of the symbiotic Fusarium sp. nov., the beetle successfully completed its lifecycle and developed from egg to fertile adults in approximately 60 days. Galleries that were produced in the PDA medium by the adults, resembled those excavated in host plant xylem under natural host colonization conditions. Euwallacea aff. fornicata from avocado in Israel was not able to survive when fed with F. ambrosium but resulted in approximately 25 % mortality when fed on F. solani; both isolates originated from infected tea. Likewise, the larvae of E. fornicatus from tea in Sri Lanka, were not able to survive or complete their lifecycle when supplied with a feed of the Fusarium sp. nov. isolated from avocado in Israel. Isolates of two other Fusaria, F. mangiferae from mango and F. oxysporum f. sp. melonis from melon, were not able to support development or survival of the beetle larvae from avocado from Israel, using the same Petri dish rearing method. This indicates that the Fusarium sp. nov. isolate from avocado is obligately required for the survival and development of Euwallacea aff. fornicata currently occurring in Israel, affecting this crop and additional hosts. The usefulness of the Petri dish assay to study the interactions between ambrosia beetles and their fungal symbionts is discussed.