An innovative approach for Ambrosia beetle control:
Phoretic mites as microbial biocontrol vectors

By Steffan Hansen, Marielle Berto, Davina Saccaggi, and Francois Roets

Ambrosia beetles (Scolytinae and Platypodinae) are borers that feed on specialized fungi transmitted to the galleries they create in xylem tissue. While the vast majority of species are not harmful, a handful can be economically important invasive or emerging pests. While species specialized on dead wood are rarely of economic concern, some transmit fungal pathogens to living trees, and can become devastating invasive pests. Two such taxa are Xyleborus glabratus, transmitting laurel wilt (Raffeallea lauricola = Harringtonia lauricola) to Lauraceae (including commercial avocado), and Euwallacea fornicatus s. lat., transmitting ambrosial Fusarium spp. to more than 100 tree species in their invasive ranges. The ecological and economic impacts of these ambrosia beetles are enormous, and costs multi-billion USD in tree loss and removal.

Figure 1. An exposed Euwallacea fornicatus gallery, showing dark Fusarium euwallaceae growth, in an ornamental Acer tree, Stellenbosch, South Africa.

Figure 2. Inactive Euwallacea fornicatus galleries in the xylem of a killed tree 

Integrated pest management options of ambrosia beetles are currently limited; with pesticides/fungicides applications being impractical, expensive, or with large potential for causing tree harm through injection holes. Physical control through sanitation of infested material is an important component in control. Microbial biological control (with microbial biological control agents, MCAs) typically focuses on entomopathogens (like the fungi Beauveria spp. and Metarhizium spp.) targeting the beetles themselves, and/or competitive organisms (like Trichoderma spp. fungi) that outcompete the ambrosial fungi that the beetles rely on for food. Various Trichoderma species are already marketed as biological control agents for the management of wood pathogens, and Beauveria and Metarhizium species as biological control agents of insect pests. These agents however suffer the same limitations as conventional chemical applications, in that the target organisms (ambrosia beetles and ambrosia fungi) are located deep in galleries in host tree wood. 

Phoretic mites associated with ambrosia beetles could potentially provide an answer in transmitting MCAs into ambrosia beetle galleries. Phoretic mites have been shown capable of transmitting fatal doses of entomopathogenic fungi conidia to the pales weevil, Hylobius pales (Schabel, 1981). Many of the mites associated with bark and ambrosia beetles are fungus feeders. Fungal feeding mites can have direct impact on fungi through grazing or microbial secretions in their saliva, and can also act as vectors for mycopathogens or antagonistic organisms.

Our two study groups (at the University of Stellenbosch, South Africa, and the University of Florida, USA) are currently investigating this innovative biological pest control approach. Stellenbosch University is focusing mainly on Euwallacea fornicatus causing Fusarium dieback on a variety of trees and agricultural crops in South Africa; the University of Florida on a variety of ambrosia beetles transmitting laurel wilt to avocados and other Lauraceae.

Figure 3. A Histiogaster arborsignis sp. group hypopus; these mites are commonly found in association with bark and ambrosia beetles, and other wood boring insects. 

Figure 4. Fusarium euwallaceae (laboratory food source) macro conidia clinging to the ventral plate of a Trichouropoda sp. deutonymph, found in association with Euwallacea fornicatus in South Africa. 

Mites associated with target ambrosia beetles are collected and identified, reared in the laboratory, and assessed for potential in transmitting MCAs to ambrosia beetles and their symbiotic fungi. Both study groups have identified the Histiogaster arborsignis species group, a phoretic, fungal feeding Acaridae mite group, as a common ambrosia beetle associate. Histiogaster arborsignis are easy to rear in the laboratory, and forms a hardy dispersal stage or ‘hypopus’. Feeding and reproduction on a variety of ambrosial and canker fungi have been demonstrated, and the South African samples have been able to feed and reproduce on the insect-killing Beauveria bassiana and fungus-killing Trichoderma atroviridae. Two species of Trichouropoda, an omnivorous mesostigmatan mite genus commonly associated with bark beetles, have been obtained from South African ambrosia beetles (E. fornicatus, and a Playtpus sp. respectively). Both species are easily cultured on Fusarium euwallaceae inoculated onto moist barley, and apart from its ability to transmit MCAs, it will be investigated as a potential predator of ambrosia beetle eggs and larvae. Further trials in the biological control potential of these mites are currently underway.

Figure 5. Histiogaster arbosignis sp. group hypopi, and adult female Proctolealaps sp., converging around an active Euwallacea fornicatus gallery. South Africa. 

Acknowledgements

Our research was funded by Hortgro Science, SAMAC and SAPPA (S. Hansen et al.), and USDA and University of Florida (M. Berto et al.). We wish to thank Edward Uekermann, Pavel Klimov and Wayne Knee for assistance in mite identification.


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Last updated October 14, 2024