Evaluation of Mulches, Soil Applied Fabric and Sub-Irrigation for Control of Adult Diaprepes abbreviates L. Emergence From Soil

The Diaprepes root weevil, Diaprepes abbreviates L. (Coleoptera:Curculionidae) was discovered as the cause of an early decline and death of citrus trees and some ornamental plants along coastal Orange and Los Angeles Counties in 2005 and San Diego County in 2006.  Quarantine was established and an attempt to eradicate this invasive pest was initiated in 2006, but we discovered that soil applications of the pesticides bifenthrin and imidacloprid, and entomopathic nematodes did little to control Diaprepes larvae in the clay soils along the coast.  The California Dept. of Food and Agriculture (CDFA) determined that this pest could not be eradicated and the quarantine was discontinued in 2009.  Research efforts to control this pest are now focused on cultural and other biological control methods.  We conducted a study in a six yr old lemon grove in 2009 and 2010 to evaluate the effect of blocking adult emergence from soil by using a thick mulch layer, a small-meshed landscape fabric and a dry soil barrier (created by sub-irrigation). The non-treated replicates served as the controls. We hypothesized that at least one of these barriers could interfere with the life cycle of this pest and eventually reduce the adult populations  in the foliage of lemons. The trial was set up with 9 tree units for each of the four treatments (with the center tree as the data tree, monitored weekly by Tedders traps), this was replicated four times.  The mulch treatment consisted of a six in (21 cm) layer of composted wood chips, the landscape fabric consisted of a 144 sq ft of small meshed fabric (Weed-X Pro) under each tree, and the dry soil barrier treatment was created by installing three underground drippers under each tree (output matched the 15 gal//hr applied by micro-sprinklers on the control trees). Almost 100% reduction of adult emergence was achieved when the landscape fabric was kept intact (2009) but emergence occurred in 2010 when the fabric was torn by grove operations.  Adult emergence was reduced to about 70% in the sub-irrigation 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 and this resulted in greater adult emergence (compared to the controls) during 2009 and 2010.  Landscape fabric appears to be successful as a cultural control measure for Diaprepes adult emergence from soil.