Microsporidia play the "bad guys" in a biological control program

The hemlock woolly adelgid (HWA), Adeges tsugae,is an invasive hemipteran herbivore that is devastating hemlock stands in the eastern US where few natural enemies are present to suppress populations. The steady destruction of these shade tolerant evergreens is changing the structure and ecology of many eastern forests. The US Forest Service and its research partners have cooperated since the 1990s to develop a biological control program for HWA. A major focus is the evaluation and mass rearing of predatory beetles imported from China and Japan,and from the Pacific Northwest. Problems with mass rearing initiated evaluation of beetle colonies in a number of rearing facilities. Microsporidia were implicated as mortality factors in a number of colonies and in some colonies reached high prevalence levels, up to 50%, within 2-3 years. At least three species of microsporidia were recovered, two from the coccinellid Sasajiscymnus tsugae from Japan,and one from the derodontid Laricobius nigrinus, a native predator of the western North American HWAlineage. Laboratory host specificity tests of Tubilinosema sp. showed that three other beetle species, all putative biological control agents of HWA, were susceptible to infection, including the coccinellid Scymnus sinuanodulus, the native L. nigrinus, and Laricobius osakensis, native to Japan. In addition, we found microsporidian infections in S. tsugaeand L. nigrinus, as well as a different microsporidian species in the native pine bark adelgid predator Laricobius rubidus in eastern release sites. Infection prevalence was low in release sites for two seasons of monitoring but microsporidia can seriously compromise mass rearing efforts as well as compromise the success of field-released predators.