Weevil Watch A


lfalfa


DR. LEE TOWNSEND LEXINGTON, KY.


populations are continuing


larvae (less


weevil to


develop with mostly small


than 1/8 inches long) (Figure 1) col- lected in sweep sam- ples taken from 4- to


6-inch alfalfa in three Fayette county fields on March 24, 2017. While not an effective assessment tool


to determine the need for an insecticide treatment, sweep net collections can give some useful information about weevil populations (Figure 2). Samples taken on March 24 show the range and comparative size of larval numbers in three fields on the UK Spindletop Ex- periment Station Farm. Here are some points to consider • Fields 1 and 2 were adjacent estab-


lished fields with good stands of alfalfa. While there were no readily apparent


differences, the total larvae per 30 sweeps from each field were 17 and 37, respectively. This emphasizes the need to check each field because insect in- festations may vary from field to field. • Most of the weevil larvae collected


were very small (less than 1/8-inch long). It takes close inspection to catch the pin-holes chewed into tender tip fo- liage by these small larvae. • The damage potential from alfalfa


weevil is based on larval numbers and plant height. Stem length is still rela- tively short so fewer grubs are needed to affect yield and qual- ity. Taller plants can tolerate a heavier weevil load. Watch al- falfa while it is still in the vulnerable range. Field 3 Field 3 is a very dif-


ferent situation. This alfalfa stand has de- clined significantly


(Above) Figure 2. Alfalfa weevil larvae per 30 sweeps and mean stem length from three Fayette County fields sampled March 24, 2017.


(left) Figure 1. Mainly small alfalfa weevil larvae have been collected in sweep nets. Photo: Lee Townsend, UK


and purple deadnettle is thriv- ing. The sparse stand may not have attracted many adult weevils during their fall return flight to alfalfa. Continue the Weevil Watch The next 2 weeks should tell


the tale in terms of the 2017 weevil population. Continue the weevil watch. While the 30-stem sampling method provides an excellent means of assess- ing the need for treatment, the per- centage of infested tips provides an alternative method. This approach rec- ommends treatment if 25 percent to 50 percent of the alfalfa tips have feeding


damage and there are two or more live larvae per stem. If weevil numbers or damage are suf-


ficient to warrant an insecticide appli- cation,


the current degreeday


accumulation for your location can help to determine a rate. This informa- tion is available at Calculating Degree Days website. Use a medium residual (mid-rate) insecticide if the accumula- tion is in the range 276 to 375. Use a short residual (low rate) if the accumu- lation is 376 or greater but harvest in not imminent.


∆


DR. LEE TOWNSEND: Extension Ento- mologist, University of Kentucky


April 7, 2017 / MidAmerica Farmer Grower • 17


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