Author: Richard Southamer

The polyphagous shot hole borer (Euwallacea fornicatus) is an invasive species in California that has been present at least since 2003. It is an ambrosia beetle that lives in close symbiosis with several fungi on which both the adults and the offspring feed. The pest status of this insect and associated fungi was not realized until in 2012 a backyard avocado tree in South Gate was found to be infected by both beetle and its fungi by Dr. A. Eskalen (Department of Plant Pathology and Microbiology, UCR). Using DNA finger prints Dr. R. Stouthamer’s lab at UCR determined that the PSHB invading California shared a DNA finger print with the invasion that was taking place in Israel where this beetle/fungus combination caused severe damage to avocado. In addition the DNA finger printing showed that the beetle in California and Israel differed from known E. fornicatus populations found in Hawaii, Florida, Sri Lanka, southern Thailand, Indonesia and Australia. The difference in DNA sequences between the California invasion and the beetles found in the aforementioned locations was such that it would be highly unlikely that they were the same species. The only collections that we had that showed larger similarity but not identity with our beetle came from the northern part of Thailand. Subsequently, additional samples showed that our beetle also occurred in Okinawa (Japan) but again these beetles were not identical to ours. It was not until we received a large sample from Viet Nam that we found beetles having DNA fingerprints identical to ours, in addition to beetles with slight variants of the DNA finger print were found in Viet Nam was well, indicating that Viet Nam most likely belong to the native area of this beetle. Native areas of insects will generally contain the largest amount of genetic variation in both beetle and its symbionts, but more importantly from our point of view, will also contain the most diverse natural enemy complex. Our cooperator in Viet Nam had already found several species of flies that he assumed were predaceous on the beetle. 

In Viet Nam we were taken around by our cooperator Dr. Thu and people from his department of the Forest Protection Research Center of the Vietnamese Academy of Forest Sciences. Our trip in Viet Nam consisted of going out to different localities where the PSHB had been found by Dr. Thu and his team consisting mainly of Acacia and Cinnamon plantations, and upon our request areas where avocado was grown commercially. Our mode of action on these field trips would be to find PSHB infested trees, cut parts out of the tree, take them to the lab and dissect the log there to 1. Collect all insects and mites found in the galleries, and 2. Make fungal preparations from the galleries to determine the fungal composition and to collect potential antagonists to the harmful fungi that the beetle carries. Taking the logs apart meant cutting them first into smaller sections of about 4 inches lengths and followed by cutting those sections into smaller slices. This allowed us to look into the galleries of the beetles and to remove any insect or mite specimens that were present there.