The objective of the “predator cocktail” strategy is to achieve year-round predation on the hemlock woolly adelgid (HWA). All HWA predator beetle candidates are “specialist” insects that feed almost exclusively on adelgids, and all are subject to a seasonal diapause in which they suspend activity in response to environmental conditions, especially temperature. So year-round predation would require two or more predators with non-overlapping diapause “seasons”. And this could be best accomplished by finding a winter-feeding predator (with Summer diapause) to supplement our summer-feeding predator (with Winter diapause) from Japan. For more on biological control of HWA, go to Biocontrol.
Summer-feeding HWA Predators
Summer-feeding predators are required to establish control of both Spring and Summer HWA reproductive cycles. The first choice for a summer HWA predator beetle is Sasajiscymnus tsugae (Sasi or St for short), which is the native ladybug predator for our HWA “import” from southern Japan. Sasi has been widely released by US Forest Service on federal lands (National Parks and Forests) and some state lands in the eastern US. And this predator is commercially available during the Spring release season to private landowners from a rearing laboratory in PA (Tree-Savers.com).
Sasi experiences a cold-temperature-induced winter diapause in which it rests during the coldest part of the year (~ mid-November to mid-February in the southern Appalachian range). This winter diapause gives it considerable cold tolerance, and its summer feeding encompasses both HWA reproductive cycles (sistens & progrediens), which can extend through July. Plus, this is the only HWA predator that produces multiple generations during its reproductive season and in which adult beetles survive to breed over multiple seasons.
Winter-Feeding HWA Predators
Winter-feeding predators can be a useful addition to increase predation on the Spring HWA reproductive cycle. The search for a winter-feeding predator beetle (with Summer diapause) typically leads to members of the Laricobius genus: the native Laricobius rubidus and two non-native species Laricobius nigrinus and Laricobius osakensis. Each of these beetles meets the criteria of an adelgid-specialist predator with a Summer (mid-May to mid-October) diapause. And each has a reproductive cycle that has thus far defied efforts to laboratory-rear Laricobius predator beetles on a commercial scale. However, there are important differences between the three Laricobius species.
Laricobius rubidus is a native derodontid adelgid predator beetle that is widely distributed throughout the eastern US. There it feeds on the pine bark adelgid (Pineus strobi), also a native insect that is found on the Eastern White Pine (Pinus strobus). White Pine trees in the wild do not appear to be seriously affected by pine bark adelgid (PBA) infestations, but this can be a serious problem on stressed saplings. However, the biological role of L rubidus in this tolerance is not well understood. What is understood is that while L rubidus will feed on HWA, it prefers PBA for both feeding and reproduction. And so our native Laricobius rubidus has not proved an effective control agent for the imported hemlock woolly adelgid in the eastern US.
Laricobius nigrinus is a predator beetle from the North American Pacific Northwest and is very closely related to L rubidus. Its preferred food in warmer coastal areas is the Pacific HWA, which is a different (genetically distinct) organism from the (Southern Japan-origin) HWA found in the eastern US. The Pacific HWA is not capable of attacking healthy western hemlocks (Tsuga heterophylla) in the wild, but typically is found on hemlocks that have been damaged or stressed (often by human actions). As a result, the Pacific HWA and its L nigrinus predator have a geographic distribution that is mostly limited to human-occupied areas. And the “wild” collection supply for this predator species is very restricted, while efforts to field-rear this predator in the eastern US has been hampered by subzero winter temperatures
An inland cold-hardy population of L. nigrinus has also been identified in Idaho, where it is collected from western white pine adelgid and western hemlock HWA sites. But its field overwintering success in the eastern US is questionable.
Laricobius osakensis is a predator beetle that was discovered feeding on our HWA import in its native range in Southern Japan. In that area, it joins with S tsugae in protecting the southern Japanese hemlock (Tsuga seiboldii) from the same HWA that threatens our native hemlocks in the eastern US. This predator beetle has only recently received USDA approval for release in the eastern US, and is not yet available to private landowners.
Our Japanese HWA import has obviously not been controlled by the resident Laricobius rubidus. So the competition for the best winter-feeding addition to a predator cocktail with the summer-feeding Sasajiscymnus tsugae is between Laricobius nigrinus from the Pacific NW and Laricobius osakensis from Japan. Neither is native to the eastern US, but both are adelgid “specialist” predators which should not represent a threat to other native insects or plants. One glaring exception to this statement is the fact that L nigrinus is so closely related to L rubidus that the two species can hybridize, with unknown long term implications for either species! (Does anyone else wonder why this hybridization between an introduced and a native species is not a concern at USDA?)
Researchers at Virginia Tech have determined that L nigrinus and L osakensis have very similar predation and reproductive rates. And both species are in diapause during the summer HWA reproductive cycle, which requires that they be paired with a summer-feeding predator for effective HWA biological control. But L osakensis appears to have a higher tolerance for cold temperatures during the winter feeding period. In fact, L nigrinus from Pacific NW coastal sources does not appear to survive winter temperatures in most northern parts of the eastern US. And the 2014 Polar vortex event reportedly damaged naturalized L nigrinus colonies as far south as Georgia. However, the inland L nigrinus colony in Idaho should be comparable in cold tolerance with L osakensis.
Sources for Winter-feeding Predators
Unfortunately, the inability to dependably rear the Laricobius species in laboratory conditions has limited the availability of non-native Laricobius to private landowners. In fact, field collections of L. nigrinus in the Pacific NW remain the major source for Lari releases in the eastern US.
L nigrinus: For over a decade, Dr. Richard McDonald (www.drmcbug.com) has played a major role in collecting the “tender” strain of L nigrinus from coastal areas of the Pacific NW. And, winter weather permitting, he hopes to supply L nigrinus from NC field insectaries to private landowners.
There does not appear to be a source for the “hardy” inland strain of L nigrinus from Idaho that can supply these to private landowners.
L osakensis: This species has just recently cleared APHIS restrictions on US release but is apparently not yet available to private landowners.