Cowpea curculio in southern peas
Management of overwintering cowpea curculio in southern peas
Cowpea curculio, Chalcodermus aeneus (Boheman), is the key insect pest of southern peas or cowpea, Vigna unguiculata (L.), in Georgia and elsewhere in the southeastern USA. Lack of control of this pest has caused localized collapses in southern pea production in Georgia. Since there are no current feasible control options for this insect pest, we have been exploring more its biology to see if there are periods during the year to take advantage of weak points in its annual cycle in Georgia fields. We just recently documented evidence for diapause in this species. This could help us to predict when in the fall season that this weevil is not included to oviposit and therefore cause damage to the crop during that specific time period. We also know now that the soil phase may be more susceptible to biological control.
Southern pea acreage in Georgia was 86,500 acres in 1951 and declined to a low of 4,311 acres in 1997. The inability to consistently control this cowpea curculio was one of the reasons for the decline. By 2014, the acreage, which had started to increase, again collapsed to 4,966 acres (valued at $5,170,111). There are currently no labeled insecticides that adequately control this pest. Labeled pyrethroid insecticides are only partially effective, mainly when curculio populations are low. When the fall population of curculios experience more overwintering death, the overall population level of this insect declines. Thus, we need to consistently reduce the overwintering fall populations of cucurlio, if we are to stop further declines in southern pea acreage in curculio infested regions of the state. Since the curculio enters the soil before overwintering, it is important to target the soil for control.
We conducted a soil pesticide treatment tests to target the soil phase of the fall (and summer) populations of cowpea curculio. We evaluated several synthetic and biologically based pesticides applied after the harvest of southern peas, when curculio grubs are entering the ground to pupate and eventually emerge from the soil as adults. We also are in the process of documenting diapause, a non-reproductive state of the insect which puts reserves into fat tissue, in the curculio in the fall populations of curculio as compared to the summer populations. If it is present in this insect, then this would explain how the weevil can be so long lived during the winter months. It also will indicate when in the fall that the population causes less damage to the cowpea crop.
We were able to document a significant reduction in overwintering curculios with certain soil treatments. A maximum of 58% reduction of average curculio emergence was measured across the treatments over the entire duration of the test. The Lorsban advanced drench, which is a more traditional pre-plant soil treatment provided the most effective control of the chemical insecticides used compared against the check with a 43% reduction in emergence. Brigade, a pyrethroid, which is traditionally less effective on cowpea curculio performed the worst of all the treatments applied. The best treatment in terms of reduced weevil emergence was the Beauvera bassiana (Botanigard); we saw a 58% reduction in emergence in plots treated with B.bassiana when compared against the check. We believe that increasing the number of emergence traps per treatment plot might have reduced the variance enough to provide a significant treatment effect. Heterorhabditis bacteriophora (Nemseek) performed on par with the Lorsban advanced drench and slightly better than the other two species of nematodes used Steinernema carpocapsae (Nemattack) and Steinernema feltiae (Nemattack). We are currently re-testing several of the best soil treatments this fall. The documentation of diapause is still on-going, but seems to support the diapause theory. A Masters graduate student is completing her thesis with this project.