The soapberry borer (Agrilus prionurus) is native to Mexico. It was first detected in Texas in 2003. The beetle was probably present for several years before its detection; there are some reports of beetle damage are as early as 1998 (Billings and Pase 2009; Anonymous 2007).
The only known host is Western Soapberry (Sapindus saponaria L. var. drummondii (Hook. and Arn.) L.D. Benson). Soapberry is a medium-sized, drought-hardy native tree that occurs in scattered patterns along creeks. It is occasionally planted in native landscapes. (Merchant 2006; Billings and Pase 2009). Soapberry’s range extends from northern Mexico to Missouri, and west to Arizona (Billings and Pase 2009).
Western soapberry is an indicator species for riparian habitats in parts of the American southwest. Trees in the genus Sapindus are larval food-plants of the Soapberry Hairstreak Butterfly, Phaeostrymon alcestis. Its fruits ripen in September or October and persist on the tree until late winter or spring. The tree is valued in horticulture for its colorful fall foliage and persistence in dry, windy, sites. The fruits were formerly used to make soap (they contain ~37% saponin). The heavy, strong, close-grained wood splits into thin strips that have been used in basketry (Billings, Grosman, and Pase III 2014).
Scientists did not know whether the more eastern variety, S. saponaria L. var. saponaria (Wingleaf Soapberry), which occurs from Mississippi to Florida, would be a suitable host (Billings, Grosman, and Pase III 2014).
Improved detection and perhaps rapid spread led to multiple reports in the next decade: in January 2009 the beetle was reported in 18 counties. Fifteen new counties were added in 2009, nine counties in 2010, eight new counties in 2011. Between 2011 and December 2013, only two additional counties were added. The total then was 52 counties (Billings, Grosman, and Pase III 2014). The slowdown in the early 2010s was accompanied by fewer reports of dying Soapberry trees from landowners. In addition, many of the control trees designated in 2009 as part of studies appeared to be recovering. Billings attributes these changes to severe freezes in 2010 and 2011 and the growing populations of natural enemies (Billings, pers. comm. December 2014). At the time he was uncertain whether populations of the beetle would return to previous levels and continue expanding their range (Billings, Grosman, and Pase III 2014). There is no more recent data on the beetle on the internet.
The latest reports of infestations in Wichita, Roberts, and Grayson counties meant the beetle had reached the border with Oklahoma (the active infestation in Grayson Co. was only ~ 30 miles from the border) (Billings, Grosman, and Pase III 2014). Billings urged Oklahoma officials to be on lookout (Billings and Smith, date unavailable). There is no report of detections in that state.
Available data indicate that the beetle’s attack kills all sizes of soapberry trees larger than two inches dbh (Billings and Pase 2009).
The Texas Forest Service (TFS) undertook several initiatives to track the outbreak. In 2009, TFS sought input through a volunteer survey questionnaire sent to county extension agents and administrators of state parks and wildlife management areas, and posted on websites managed by the Texas invasive species council, Texas Forest Service, and International Society of Arborists Texas Chapter. TFS also tested purple panel traps developed to detect the congener emerald ash borer, but found them not to be very effective (Billings and Pase 2009).
TFS also began studies on control methods, specifically the efficacy of systemic pesticides (emamectin benzoate) injected into trees before infestation (Billings and Pase 2009). Emergence studies indicated that the beetle has one generation per year in Texas (Billings and Pase 2009).
Project CAPTURE
USFS scientists and managers developed a conservation priority-setting framework for forest tree species at risk from pest and pathogens as well as other threats. The Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation) uses FIA data and expert opinion to group tree species under threat by non-native pests into vulnerability classes and specify appropriate management and conservation strategies. The scientists prioritized 419 tree species native to the North American continent. The analysis identified 15 taxonomic groups requiring the most immediate conservation intervention because of the tree species’ exposure to an extrinsic threat, their sensitivity to the threat, and their ability to adapt to it. Each of these 15 most vulnerable species, and several additional species, should be the focus of both a comprehensive gene conservation program and a genetic resistance screening and development effort. GSOB is not known to be a threat to any of these 15 most vulnerable species.
Sources
Anonymous. 2007. Past Chapter President, Jerry Pulley Credited with New US Record. In the Shade (Newsletter of ISA Texas Chapter, Vol. 30, No. 5 (March 2007)
Billings, R.F. personal communication, December 2014.
Billings, R. F. and H. A. Pase III, Texas Forest Service. No Date. Soapberry Borer, Agrilus prionurus (Coleoptera: Buprestidae) An Exotic Pest Recently Detected in Texas
Billings, R. F. and H. A. (Joe) Pase III, Texas Forest Service. 2009. Soapberry Borer Infestations Found in 33 Counties in Texas
Billings, R.F., Do.M. Grosman, and H.A. Pase III. 2014. Soapberry Borer, Agrilus prionurus (Coleoptera: Buprestidae): An Exotic Pest Threatens Western Soapberry in Texas. Proceedings of the 5th Big Thicket Science Conference: Changing Landscapes & Changing Climate. 2014 Southeastern Naturalist 13 (Special Issue 5):105–116
Michael Merchant. 2006. TX A&M University fact sheet. Texas A&M University Research and Extension Center, Dallas;
Potter, K.M., Escanferla, M.E., Jetton, R.M., Man, G., Crane, B.S., Prioritizing the conservation needs of US tree spp: Evaluating vulnerability to forest insect and disease threats, Global Ecology and Conservation (2019), doi: https://doi.org/10.1016/