[Image: diaprepes weevil adult]The West Indian sugarcane stalk borer, Diaprepes abbreviatus, is an important crop pest that has now invaded Louisiana. This insect is native to the Caribbean islands where it is a significant pest of citrus and sugarcane production in Puerto Rico. D. abbreviatus was first found in Florida in 1964. It may have entered the United States on ornamental plants imported from Puerto Rico. D. abbreviatus is now a widespread pest of agricultural production in Florida at a cost of millions of dollars annually in control. Root weevils have recently been detected in the Rio Grande Valley of Texas. D. abbreviatus was found in Citrus City, Plaquemines Parish, Louisiana, in March 2008. The specimen was reported to the Louisiana Department of Agriculture and Forestry by a county agent in November 2008 after being delivered by a citrus producer. The initial find was two pairs of mating adults on the vegetation of trees. Since time has passed, the insects have now completed multiple generations in this grove, and recently (July 2009) a large flush of adults was reported.
Life cycle
The adults emerge throughout the year, but most emergence occurs from May to August. Emergences also follow rain events. We do not know the exact timing of emergence in Louisiana because we have just begun monitoring the insect. Adults feed on vegetation, causing notching damage on leaves. It is difficult to distinguish this damage from other foliage feeders, such as grass hoppers or orange dog caterpillars. Adults will mate on the vegetation, and then mated females attach eggs to leaves. Sometimes two leaves will be glued together with an egg mass in between. Larvae hatch out of eggs, drop to the soil surface, burrow down into the soil and commence feeding on roots. Damage caused by root feeding will cause a gradual decline in tree health and also increase susceptibility to phytopthora root rot damage. To confirm the presence of larvae on the roots, declining trees need to be removed from the ground and the roots carefully examined for the presence of grubs.
Management and control
In Florida, root weevils are managed using an integrated pest management approach that includes cultural, chemical and biological controls. The biological control option utilizes nematodes that can suppress larval populations. Unfortunately, these nematodes require a sandy soil type to survive and thrive. The soil type in Plaquemines Parish is not sandy, so this is not an option for our management.
Cultural control involves plac[Image: weevil grub]ing restrictions on movement of potted citrus and ornamental plants. Movement in pots is probably the most common way D. abbreviatus is moved throughout a state and from state to state. Because larvae are feeding below the soil surface, they are difficult to find. It is critically important that potted material is not moved from infested areas into non-infested areas. It is also important to obey all interstate restrictions on plant movement to prevent the entry of this pest into other regions of Louisiana.
Chemical control involves a three-fold approach that will target egg, larval and adult life stages of the insect. Micromite 80SG is an egg sterilant that can be used to decrease the viability of eggs produced by adults. Controlling the adult stage during mass emergences appears to be the best method of control. Insecticides that can be used for adult control on vegetation include Danitol 2.4ec, Kryocide, Orthene 97 (non-bearing citrus only) and various formulations of Sevin. Be sure to adjust water pH into the range of 5.5 to 6.5 if you plan to use an organophosphate or carbamate, such as Orthene or Sevin. Please be sure to read and follow all label instructions. Chemical barriers also are used to kill larvae as they burrow in the soil and to kill adults as they emerge from the soil. It is critical to maintain a vegetation-free soil surface if chemical barriers will be used for management. Bifenthrin formulations can be used as a soil barrier – these include Brigade WSB.
Host range
Root weevils have a very broad host range, which includes citrus, sugarcane, sweet potatoes and a variety of ornamental plants (270 are known). It is critically important that all individuals associated with these industries familiarize themselves with how to identify this insect. Vigilant monitoring for spread will be critical to maintain this insect at a manageable population level in Louisiana. If you find an insect that looks like Diaprepes root weevil, please collect a sample and report immediately to your local LSU AgCenter parish office or Louisiana Department of Agricultural and Forestry (LDAF).