Evaluation of Insecticides, Entomopathogenic Nematodes, and Physical Soil Barriers for Control of Diaprepes abbreviatus (Coleoptera: Curculionidae) in Citrus.

The Diaprepes root weevil, Diaprepes abbreviatus L. (Coleoptera: Curculionidae), was discovered as the cause of early decline and death of citrus trees and ornamental plants along coastal Orange and Los Angeles Counties in 2005 and San Diego County in 2006. We established trials to evaluate the effectiveness of two tactics to manage life stages of Diaprepes root weevil that are found in the soil. The first tactic was soil applications of the pesticides bifenthrin or imidacloprid, with and without entomopathogenic nematodes, to kill larvae. The second tactic was attempting to block adult emergence from the soil using a thick wood-chip mulch layer, a small-meshed landscape fabric, and a dry soil barrier (created by subirrigation). We found that soil treatments with the two insecticides (bifenthrin and imidacloprid) at maximum label rates with and without Steinernema riobrave Cabanillas et al. at 5 million nematodes per tree significantly reduced numbers of soil-borne Diaprepes root weevil larvae in one trial compared with the control. Another entomopathogenic nematode, Heterorhabditis indica Poinar et al. applied in wax moth Galleria mellonella L. cadavers at 86 wax moth cadavers per tree was not effective. Adult emergence was reduced by almost 100% when the landscape fabric was kept intact (2009), but emergence occurred in 2010 when the fabric was torn by grove operations (60% reduction). Adult emergence was reduced to ≍70% compared with the control in the subirrigation treatment in 2009 when the summer was hot and dry, but emergence occurred in 2010 during periodic rains toward the end of summer. The mulch treatment kept the soil moist in between irrigations resulting in greater adult emergence rates compared with controls during 2009 and 2010. Health ratings of the trees were taken following several years of physical barrier treatments, and trees treated with the landscape fabric were significantly healthier (mean rating 0.9, scale 0 = healthy to 5 = dead) than those in the other treatments (mean rating of 3.6, 4.0, and 2.5 for control, mulch and subirrigation, respectively). Of the physical barrier methods studied, landscape fabric is most effective in reducing Diaprepes root weevil emergence, increases plant health, and is long lasting, but it is the most costly to install and can be damaged during harvest.