Virginia Peanuts: Worm Potential This Season, Be Vigilant
Reports are coming in about infestations of both beet armyworm and tobacco budworm in peanut fields in Georgia and South Carolina. These are pretty early reports and could signal trouble to come.
Although we get both species in this area of Virginia, it has been several years since we have experienced severe infestations of either.
Beet armyworms seem to be cyclic, occurring only once every few years, with no pattern that we understand. Moths migrate into the area from the south and are particularly attracted to pigweed species. Often, the caterpillars will be found feeding on pigweeds on field edges or wherever the weeds are growing.
They feed on pigweed plants then move to nearby crop plants. So, peanut, cotton or soybean fields bordered by a lot of pigweeds or with a lot weeds in the field are particularly vulnerable to beet armyworm infestations.
Tobacco budworm is known to be an occasional pest of cotton, peanuts and soybeans in this area. Caterpillars are easily confused with corn earworm, a different but related species. Adult moths of these species look very different, but the caterpillars are almost identical except for features that require some knowledge and good magnification.
We separate these species by holding large sized caterpillars under a microscope and inspecting the shape of the mandibular ridge the inside jaw of the mouthparts. Obviously, this is not something that can be done in the field.
Budworms are not a big threat to peanuts or soybean, as they are leaf feeders only, and it takes a lot of feeding to do economic damage. They do no more damage than corn earworm, the more common of the two species.
In peanut and soybean fields, both budworm and corn earworm can occur at the same time, but typically corn earworm is the dominant pest making up 70% or more of the total. In cotton, budworms can be troublesome as they feed on young developing squares.
Another difference between these species is the timing of the infestation. In some years we see budworm infestations early, before corn earworm. This seems to be the case this year, at least in states south of us. Corn earworm moths are starting to show up in Virginia. We are catching 10 to 20 moths per night in our local pheromone traps, which isn’t a lot compared to what we will see later in the season, but enough to result in some possible small outbreaks of worms.
Beet armyworm, tobacco budworm and corn earworm are strictly leaf feeders in peanut so any decision to treat a field should be based on:
- The number of worms per row foot.
- The degree of defoliation compared to the total foliage of the canopy.
The threshold for worms for this time of year is 4 per row foot (total, all species). If you sample using a drop cloth, after slapping plants on both sides of a 3 foot long cloth you would need to see at least 12 worms. The likeliness of actually finding this many worms is very low, and it is pretty rare to find threshold levels of worms in any peanut field.
But is always pays to be vigilant.
Check fields periodically throughout the season for pests. If after checking you find at or near thresholds of any pest, select a product that fits the need. For example, if you do find a field at threshold for a combination of beet armyworm, tobacco budworm or corn earworm, pyrethroids would not be the best choice. These species would be controlled better with non-pyrethroids like Steward, Belt, Prevathon, or Blackhawk.