Parasitism of Asian citrus psyllid by Tamarixia radiata on residential citrus in South Texas: Importance of colony size and instar composition

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), is a serious threat to the Texas citrus industry because it spreads Huanglongbing (HLB) among residential citrus and from residential neighborhoods to commercial groves.  The nymphal parasitoid, Tamarixia radiata Waterston (Hymenoptera: Eulophidae), has been extensively released for biological control of D. citri on dooryard citrus in South Texas since 2010 but currently provides insufficient control because parasitism rates are highly variable. On citrus trees, D. citri colonize young flush shoots to form discrete patches (colonies) of psyllids at various life stages.  We present findings from 2016-17 and 2019 field studies on the effects of D. citri colony size (nymph density per shoot) and instar composition on parasitism rates by T. radiata on residential citrus in South Texas.  Patterns and levels of D. citri parasitism were strongly linked to the discovery and usage of preferred host stages (4th to 5th instars) by T. radiata.  Because parasitism of 3rd or younger instars of D. citri was very rare, a 'host patch' was defined as the cluster(s) of 4th to 5th instars within a colony infesting a shoot.  Based on apparent or actual parasitism, host density was positively related to discovery of D. citri colonies but inversely related to percent parasitism by T. radiata.  The inability of T. radiata to achieve high rates of field parasitism under a wide range of host densities could explain, in part, its low impact against D. citri on residential citrus.