August 2018 Articles

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Management of Armyworms in Pastures And Hay Meadows

Management of Armyworms in Pastures And Hay Meadows Life Cycle timing: NOTE — Development rate is faster at higher temperatures (90 degrees and up)!  Adult (moth) — Ash-gray with a wingspan of about 1½ inches. Will have a white spot in center of each forewing. 10 to 14 days to complete this part of lifecycle and will lay between 1,000 and 2,000 eggs in that period.  Eggs — Laid in masses of up to 50 eggs on light-colored surfaces such as wide blade grasses, tree trunks, tree limbs and farm structures (i.e., fence posts, fence railings, buildings). Eggs are covered with the grey scales from the moth’s body, giving the egg mass a fuzzy appearance. Hatch within 2 to 5 days.  Larvae (caterpillars) — When fully grown, larvae may be up to 1½ inches long and vary in color from light green to almost black. 10 to 22 days to complete this part of lifecycle. Shorter during hotter periods and longer during cooler periods.  Pupae — Fully grown larvae burrow into the soil and form pupae. Moth will emerge in about 10 days in the heat of summer. Scouting:  The best way to avoid losing a cutting of hay to armyworms is to visit fields every three or four days and scout.  In the early morning or late afternoon get out of the vehicle, get down on your hands and knees, and look closely! Scout for armyworms by vigorously ruffling the grass and carefully counting the larvae that fall to the ground in a one-square-foot area. Do this at several locations in the field and average your results. Treatment is recommended when counts exceed three caterpillars that are one-half inch or longer per square foot.  Experienced forage producers learn to recognize the early signs of fall armyworm feeding. Newly hatched caterpillars are too small to feed all the way through a leaf blade. They begin by feeding on the underside of the leaf but leave the clear upper epidermis intact. This creates tiny “windowpanes” in the grass blades that appear silver or white. When large numbers of small larvae are just beginning to damage a field, the grass often has a subtle “frosted” appearance because of these windowpanes.  Use randomly placed light colored flags across the field to provide an attractable place for females to lay eggs. When large numbers of egg masses begin to show up on the flags, get ready to spray in 2 to 4 days.  Don’t depend solely on watching for egrets or other birds feeding in a field as an indicator of armyworm problems. If you see birds in a field, you definitely want to check out what they are feeding on, but do not use this as your only way to scout. This sign often comes too late to avoid damage because the birds do not show up until the caterpillars are big enough to attract their attention. Insecticidal Control: There are numerous insecticides that can be used for controlling fall armyworm caterpillars in perennial grass forages. It is important to read the directions for use (the label) of an insecticide carefully to make sure that the product can be applied to a particular type of forage. The label also contains information on how to mix and apply the product for maximum efficacy. In general, use higher rates of insecticides (and higher volumes of water — 20+ GPA) when the grass is thick, when armyworm populations are high, when caterpillars are large, and to get the longest residual effect. Some insecticides will kill only smaller caterpillars, so check the comments about each insecticide. Pay close attention to the number of times a particular insecticide can be applied per hay cutting and per season. Within a season, rotate between different modes of action, if possible. The following generalizations may help you match your circumstances to the right insecticide.  Insecticides with the shortest interval between last application and when a pasture can be grazed (grazing interval in parentheses): beta-cyfluthrin (0), cyfluthrin (0), chlorantraniliprole (0), diflubenzuron (0), gamma-chylothrin (0), lambda-cyhalothrin (0), methoxyfenozide (0), spinosad (0), and zeta-cypermethrin (0). On the other extreme, carbaryl (Sevin) has a 14-day grazing and harvest restriction.  Insecticides with the shortest interval between the time of application and harvest (preharvest interval): beta-cyfluthrin (0), cyfluthrin (0), chlorantraniliprole (0), zeta-cypermethrin (0), diflubenzuron (1), methomyl (3), and spinosad (3).  Insecticides with the longest residual: methoxyfenozide, diflubenzuron, and chlorantraniliprole. Insecticides with longer residuals help in outbreak years when generations of fall armyworms overlap, resulting in almost continuous egg laying. Caterpillars sprayed with methoxyfenozide or chlorantraniliprole quickly stop eating, become paralyzed, and then die with-in a few days. Caterpillars sprayed with diflubenzuron do NOT stop eating until the next time they molt. That is why it is essential that diflubenzuron be applied when the fall armyworm caterpillars are ½ inch long or less, well before they get to their last molt. Remember that about 80 percent of the total feeding is done after the last molt, in the last four days of the caterpillar stage.  Insecticides that do not require a pesticide applicators license: carbaryl, chlorantraniliprole, methoxyfenozide, and spinosad. REMEMBER: THE LABEL IS THE LAW! Always read the pesticide label before using. Tips for Application of Insecticides  Scout, Scout, Scout. Closely look at hay fields every 3 to 4 days. Find the caterpillars when they are small so you can save money by using a lower rate of the insecticide. If populations meet or exceed thresholds, apply insecticides ASAP.  Apply insecticides early or late in the day because the caterpillars are more active at these times.  Use as large a spray volume as you can (20+ gallons per acre if possible).  Never apply the insecticide in less water than is recommended on the label.  Stop the advance of caterpillars marching into a field by applying several swaths of the insecticide on either side of the front line.
Source: Pasture to Market July – August 2018 LSU AgCenter