Alfalfa Weevil Problem in 2005 By Wayne Bailey

Problems with alfalfa weevil larvae began in southern Missouri this past week with some fields requiring an insecticide application. The alfalfa weevil grows through 4 worm stages or instars with the smallest larvae hidden in plant growing points. During the past two weeks high numbers of small larvae have been dissected from the growing points of alfalfa plants in southern Missouri. Damage from 1st and 2nd instars is often hidden or sometimes seen as small holes in the leaflets forming plant growing points. Small larvae often are missed or may remain in the plant growing point when sampled by the bucket method. In contrast, 3rd and 4th instar larvae are readily observed and can significantly damage alfalfa plants by consumption of leaf tissue. Economic damage from alfalfa weevil throughout the southern region of the state is expected to increase during the next two weeks. Problems with this pest are expected to move northward as larvae emerge from eggs and begin feeding in central and north Missouri counties. Although larval numbers are high and some applications of insecticide have been needed, recent wet weather may initiate a fungal pathogen which can cause the alfalfa weevil larval population to collapse within a few days after they are infected. Spring weather has a strong influence on whether or not alfalfa weevil become a major problem this spring. Dry to normal conditions favor the survival of the alfalfa weevil larvae, whereas, wet conditions favor the fungal pathogen that often kill larvae. At this time producers in southern and central Missouri counties are encourage to scout for alfalfa weevil larvae in fields.