Parasitism of Diaphorina citri (Hemiptera: Liviidae) by Tamarixia radiata (Hymenoptera: Eulophidae) on residential citrus in Texas: Importance of colony size and instar composition

Diaphorina citri (Hemiptera: Liviidae) is an invasive citrus pest that colonizes young flush shoots and spreads huanglongbing, a lethal citrus disease. Effects of D. citri colony size (nymphs per shoot) and instar composition on parasitism rates by the nymphal ectoparasitoid, Tamarixia radiata (Hymenoptera: Eulophidae), were evaluated on residential citrus in South Texas. From 2016 to 17, D. citri colony sizes and apparent parasitism (T. radiata adult emergence) were monitored monthly on grapefruit, lemon, lime, and orange trees at 12 residential sites. In 2019, size and instar composition of colonies and actual parasitism (mummies and nymphs with parasitoid larvae or eggs) were monitored biweekly on citrus trees at a residential park. Psyllid nymphs were found year-round at these sites with highest densities from late summer to fall and lowest densities in winter. Mean apparent parasitism was independent of citrus species and ranged from 23% in April 2016 to 4% in February 2017. Mean actual parasitism ranged from 1.5% in August to 33.4% in November 2019. Because actual parasitism of 3rd or younger instars 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.

Automated summary

This study evaluated the impact of D. citri colony size and instar composition on parasitism rates by T. radiata. It was found that host density was positively related to the discovery of D. citri colonies and inversely related to the percent parasitism by T. radiata, indicating low field parasitism rates against D. citri on residential citrus.