Competition between biological control fungi and fungal symbionts of ambrosia beetles Xylosandrus crassiusculus and X. germanus (Coleoptera:Cuculionidae): mycelial interactions and impact on beetle brood production

Authors: CASTRILLO, LOUELA - Cornell University; Griggs, Michael; Vandenberg, John

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/6/2016
Publication Date: 9/7/2016
Citation: Castrillo, L.A., Griggs, M., Vandenberg, J.D. 2016. Competition between biological control fungi and fungal symbionts of ambrosia beetles Xylosandrus crassiusculus and X. germanus (Coleoptera:Cuculionidae): mycelial interactions and impact on beetle brood production. Biological Control. 103:138-146.

Interpretive Summary: The ambrosia beetles Xylosandrus crassiusculus and X. germanus are important pests of orchards and tree nurseries. Adult beetle attacks on woody stems reduces the value of nursery stocks. Each adult female beetle uses a specialized fungus to cultivate within its host tree and use as food for its offspring. The growth of this symbiotic fungus can disrupt the flow of water and nutrients in infested trees. The use of biological control agents may prove effective by targeting both beetles and their symbiotic fungi. We used a combination of fungal growth tests and beetle bioassays to examine competition between symbiotes and three biological control fungi and the impact of these fungi on beetle brood production. We found that a mycoparasitic fungus, Trichoderma harzianum, grew faster and overcame either of two symbiotic fungi. The two entomopathogenic fungi, Beauveria bassiana and Metarhizium brunneum, blocked the spread of symbionts. In complementary beetle bioassays we found that beetle galleries, the tunnels excavated by adult females for egg laying and larval develoment, in T. harzianum-treated beech stems had sparse growth of fungal symbiotes. Many of these galleries had no, or only a small number of, eggs present. We observed reduced numbers of offspring in galleries treated with any of the three biocontrol fungi. We also observed infected adults and offspring in galleries of beetles treated with wither of the entomopathogenic fungi. These results show the potential of using biological control fungi in targeting ambrosia beetle populations either directly by killing adult females and reducing their production of offspring or indirectly by suppressing symbiote growth in their galleries.

Technical Abstract: Ambrosia beetles Xylosandrus crassiusculus and X. germanus are among the most important exotic pests of orchards and nurseries in the US and are difficult to control using conventional insecticides because of their cryptic habits. The use of biological control agents may prove effective by targeting both beetles and fungal symbionts inside tree galleries: entomopathogenic fungi could be used to target beetle foundresses and their brood, or mycoparasitic fungi, e.g., Trichoderma harzianum, could be used to target their associated fungal symbionts. We used a combination of in vitro assays and beetle bioassays to examine competition between symbionts and biological control fungi and the impact of biocontrol fungi on beetle brood production. The in vitro assays showed T. harzianum outcompeted different strains of Ambrosiella roeperi and A. grosmanniae associated with X. crassiusculus and X. germanus, respectively, whether in primary or secondary resource capture assays. In contrast, entomopathogenic fungi Beauveria bassiana and Metarhizium brunneum blocked the spread of symbionts only in primary competition assays. Complementary beetle bioassays showed that beetle galleries in T. harzianum-treated beech stems had sparse symbiont growth, many with no, or only a small number, of eggs present. Brood numbers produced by foundresses in T. harzianum-treated stems were comparable to those in stems treated with either entomopathogen at the higher dosages, in which brood reduction was likely due to foundress mortality prior to laying eggs or after laying only a small number of eggs. These results show the potential of using biological control fungi in targeting ambrosia beetle populations either directly by killing foundresses and reducing brood production or indirectly by suppressing symbiont growth in their galleries.