Post-Release Evaluation of Diaphorencyrtus aligarhensis (Hymenoptera: Encyrtidae) and Tamarixia radiata (Hymenoptera: Eulophidae) for Biological Control of Diaphorina citri (Hemiptera: Liviidae) in Urban California, USA

Abstract

Diaphorencyrtus aligarhensis (Hymenoptera: Encyrtidae) was first released in California for biological control of Diaphorina citri (Hemiptera: Liviidae) in December 2014. The establishment and parasitism rates of D. aligarhensis, along with those of another introduced species, Tamarixia radiata (Hymenoptera: Eulophidae), first released in 2011, were assessed at 15 D. aligarhensis release and 24 no-release control sites over the period 2016–2018. Study sites with citrus trees that were infested with D. citri eggs, nymphs, and adults, were located in residential areas in southern California that spanned three different climatic zones: coastal, intermediate, and desert interior sites. Parasitism rates of D. aligarhensis were low, averaging 0.62% compared to 21.2% for T. radiata which had spread naturally and established widely through the study area approximately one year earlier. Recoveries of D. aligarhensis at release sites were made eight times in 2016 and 2017. Conversely, T. radiata was recovered consistently at 34 of the 39 sites surveyed. Analyses indicated that parasitism of D. citri nymphs by T. radiata exhibited delayed density-dependence with a 12-month lag associated with reductions of D. citri densities by 50%. Irrespective of the climatic zone, the highest frequency of parasitized D. citri nymphs for T. radiata was recorded during peak periods of citrus flush growth from March through June and October through November each year. The findings reported here suggest that it is unlikely D. aligarhensis has established in California and that competition from T. radiata may, in part, have contributed to establishment failure. Consequently, biological control efforts targeting D. citri in California should focus on T. radiata.