Alfalfa weevil larvae recently started showing up in Missouri, according to Wayne Bailey, University of Missouri entomologist. Some alfalfa fields in southern and central Missouri required insecticide applications. Alfalfa weevil larvae grow through four instars or larval stages on their journey from egg to adult weevil, Bailey notes. Eggs are laid inside alfalfa stems beginning in late fall, on winter days when temperatures go above 50 degrees, and in early spring. At present, numerous larvae have hatched and can be found in the upper terminal buds of alfalfa plants in fields throughout southern and central regions of the state. These larvae are likely the result of fall- or early winter-laid eggs and are often seen initially as problems on the south-facing slopes of fields. Larvae in the first and second instar stages of growth typically feed inside plant terminals where they are difficult to find and are protected from harsh spring weather conditions. Damage from these early instars is often minor and seen as "pin hole" damage on expanding alfalfa leaflets.
As temperatures warm, additional eggs will hatch, plus existing larvae will rapidly grow into third and fourth instars. These larger instars readily move about the plant and feed on alfalfa foliage. They may consume significant amounts of leaf tissue, which typically results in substantial losses in alfalfa yield and quality. Heavy defoliation also reduces alfalfa competition with weeds and may result in increased weed populations.
Alfalfa growers throughout Missouri should scout fields as problems can quickly develop. One unknown, but important, factor is whether the recent sustained rainfall over most of the state will result in the development of a fungal pathogen, Zoophthora phytonomi, in alfalfa weevil larvae. This pathogen is present most years, but infection rates are most successful in wet years, according to Bailey. Infected larvae slow their feeding activities, turn from light green to pale yellow in color and die within a few days of becoming infected. If the pathogen develops early in the season, it can decimate larval populations. Whether it does so this year in Missouri is yet to be determined.
Source: University of Missouri.