Scientists find possible biological control for redbay ambrosia beetle

Scientists at the University of Florida have discovered a few fungal biological control options that show promise for fighting the redbay ambrosia beetle. They discuss the results of their experiments in a refereed article in Biological Control.

The redbay ambrosia beetle (RAB) was first discovered in southeast Georgia in 2002. A native of Asia, the beetle transmits a pathogen that causes laurel wilt, a disease that can kill an infected tree in just a couple of years. The beetle attacks trees in the family Lauracea, which includes redbay, swampbay, sassafras and avocado.

Female RABs bore into the trunk of the chosen tree and create galleries. After 7 days of digging, they lay eggs and secrete a fungus that destroys the food and water conduction systems of the tree. The beetle uses the fungus to feed its young. Leaves wilt, and the sawdust from the beetle’s digging into the trunk leaves toothpick-like strands on the outside of the trunk. Infected trees do not lose their leaves immediately, but the dead, wilted leaves mar scenic views in parks and halts production of fruits.

Since the discovery of the beetle, avocado growers in Miami-Dade county in Florida have prepared for its infestation. In January 2010 the first RAB was discovered in the county, a little less than 12 miles from the major commercial avocado production area in Florida. Since then, over 3,000 avocado trees from 52 groves have been destroyed because of laurel wilt.

Avocado is second after citrus as an important fruit crop in Florida, with an economic worth of $54 million annually. Since the entry of the beetle into the state, avocado growers have been treating their trees with insecticides and destroying infected trees and removing them from the production area. Once the beetle has bored into the wood, however, insecticides are not effective and are extremely expensive to administer.

University of Florida specialist Daniel Carillo, along with other specialists from the University of Florida and USDA Agricultural Research Service used grants from IR-4 and the Farm Bill to test biopesticide formulations made out of three entomopathogenic fungi, which have been used successfully to battle other insect borer pests. Entomopathogenic fungi act as a parasite on other insects, often inserting eggs inside the insect, where the resulting growing fungus eventually devours the insect and kills it. Results from experiments would provide baseline data for further research.

Using sample beetles from wilting swamp bays taken from a natural area in Miami-Dade County, scientists tested three biocontrol fungi: Beauveria bassiana and two strains of Isaria fumosorosea (lfr 3581 and PFR 97). Scientists made suspensions of the fungi (strain does not dissolve in water) and dipped beetles in each fungal suspension. The research team also treated avocado logs with each suspension and then allowed the beetles to walk on the logs and bore into them.

Beetles died within being dunked into all three liquids. The ones dipped in the B. bassiana suspension died faster. Dipped beetles died at a higher rate than beetles that walked on logs.

Scientists also washed the beetles to count the number of spores that attached to the bodies. They found that B. bassiana clung to the bodies more than the other two fungal treatments, but B. bassiana is hydrophobic, meaning that it is repelled by water, while the two strains of I. fumosorosea are hydrophilic, and attracted to water. Authors of the article speculated that the I. fumosorosea spores may not have detached from the bodies and washed off into the water.

Although scientists had hoped that these treatments could prevent RAB from boring into trees and producing galleries, none of the treatments prevented the beetles from boring or forming galleries. However, some of the beetles died before they could lay eggs, so scientists now hope that fungal treatments might kill female RAB before they can lay eggs, and if they do lay eggs, that the fungus will be transferred onto the eggs.

Because beetles died after being exposed to fungal treatments on the trunk, scientists believe that sprays of fungal suspensions could be used to reduce the number of RAB generations, and ultimately prevent tree death. Because this was a laboratory study where beetles were physically moved from one place to another and may have been stressed, scientists now have to find out if there were any mitigating factors that assisted in beetle death.

Source: Carrillo, D., Dunlap, C.A., Avery, P.B., Naverrete, J., Duncan, R.E., Jackson, M.A., Behle, R.W., Cave, R.D., Crane, J., Rooney, A.P., Peña, J.E. Entomopathogenic fungi as biological control agents for the vector of the laurel wilt disease, the redbay ambrosia beetle, Xyleborus glabratus (Coleoptera: Curculionidae). Biological Control. 81 (2015) 44-50.

This article was also featured by Entomology Today.

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