Local Crop Planting Systems Enhance Insecticide-Mediated Displacement of Two Invasive Leafminer Fly

Abstract

Liriomyza sativae and L. trifolii are highly invasive leafminer pests of vegetable crops that have invaded southern China in recent years. Liriomyza sativae was the first of these species to invade China, but it is now being displaced by L. trifolii. The rate and extent of this displacement vary across southern China. In Hainan, monocultures of highly valuable cowpea are planted and treated extensively with insecticides in attempts to control leafminer damage. In Guangdong, cowpea fields are interspersed with other less valuable crops, such as towel gourd (Luffa cylindrica), which receive significantly fewer insecticide applications than cowpea. To determine how differences in cropping systems influence the Liriomyza species composition, we conducted field trials in 2011 and 2012 in Guangdong where both species were present. We replicated conditions in Hainan by planting cowpea monocultures that were isolated from other agricultural fields, and we replicated conditions in Guangdong by planting cowpea in a mixed crop environment with towel gourd planted in neighboring plots. We then compared leafminer populations in cowpea treated with the insecticide avermectin and untreated cowpea. We also monitored leafminer populations in the untreated towel gourd. Untreated cowpea and towel gourd had comparatively low proportions of L. trifolii, which remained relatively stable over the course of each season. Avermectin applications led to increases in the proportions of L. trifolii, and after three weekly applications populations were >95% L. trifolii in both crop systems. However, the rate of change and persistence of L. trifolii in the mixed crop system were less than in the monocrop. These results indicate that L. trifolii is much less susceptible to avermectin than is L. sativae. Further, L. sativae was able to persist in the untreated towel gourd, which probably enabled it to recolonize treated cowpea.

A decade of leafminer invasion in China: lessons learned

“…Consequently, temperature‐mediated effects may contribute to the current ongoing displacement of L. sativae by L. trifolii in warm climatic areas of China . Furthermore, the competitive balance between these species can also be altered to the advantage of one species or the other by the general cropping system, with L. sativae being more susceptible to displacement in intensively managed cowpea crops, especially when grown as a monoculture . L. sativae appears to persist in more diverse host plant communities, where it has greater reproductive success than L. trifolii .…”

“…L. sativae appears to persist in more diverse host plant communities, where it has greater reproductive success than L. trifolii . Given the generally greater pest problems experienced with L. trifolii compared with L. sativae in southern China, maintaining untreated hosts that are better suited to L. sativae may increase the likelihood of its persistence in the presence of L. trifolii and aid in improved leafminer management …”

“…8 Furthermore, the competitive balance between these species can also be altered to the advantage of one species or the other by the general cropping system, with L. sativae being more susceptible to displacement in intensively managed cowpea crops, especially when grown as a monoculture. 31 L. sativae appears to persist in more diverse host plant communities, where it has greater reproductive success than L. trifolii. Given the generally greater pest problems experienced with L. trifolii compared with L. sativae in southern China, maintaining untreated hosts that are better suited to L. sativae may increase the likelihood of its persistence in the presence of L. trifolii and aid in improved leafminer management.…”

“…Given the generally greater pest problems experienced with L. trifolii compared with L. sativae in southern China, maintaining untreated hosts that are better suited to L. sativae may increase the likelihood of its persistence in the presence of L. trifolii and aid in improved leafminer management. 31 In summary, the displacement of L. sativae by L. trifolii in southern China may be attributed to greater insecticide tolerance of L. trifolii, high environmental temperatures and specific cropping systems that favour L. trifolii, the more recent invader. A comparison of events in China, Japan and the United States demonstrates that interactions between L. sativae and L. trifolii are complex, and that competitive outcomes are likely affected by additional factors, such as fecundity, effects of natural enemies and interspecific hybridisation.…”

Pesticide-mediated interspecific competition between local and invasive thrips pests

“…Research has demonstrated that differential susceptibility to insecticides may increase the likelihood of one species being replaced by another, thus driving changes in population demographics171819. Two of the most commonly applied insecticides for management of thrips in China are avermectin and beta-cypermethrin15.…”