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Tuesday, May 17, 2011

What to watch for: Tobacco budworm

First instar tobacco budworm larva in eastern North Carolina tobacco. Photo via Loren Fisher.

Tobacco budworm populations developing
Tobacco budworm (Heliothis virescens) larvae are beginning to show up in North Carolina tobacco fields. At our research plots at the Upper Coastal Plain Research Station, Rocky Mount last week, we found several infested plants, and yesterday at the Lower Coastal Plain Research Station, Kinston, plots were between 0-10% infestation in most of our trials. The vast majority of the larvae present were recently hatched 1st instars, although a few 2nd instars were also present. This means that egg laying occurred fairly recently and that growers will likely begin to see tobacco budworm larvae in their fields soon. In fact, a consultant called Loren Fisher and I last week to report that he and his staff were seeing budworm larvae in fields in eastern NC. Populations in these fields were near treatment thresholds (10%). As always when I discuss treatment thresholds for tobacco budworm, it is important to understand that these thresholds are very conservative, meaning that yield loss has rarely been documented due to budworm feeding, even at much higher infestation rates than 10%.

Campoletis sonorensis wasp searching for young tobacco budworm larvae, Kinston, NC. Photo: HJB

Natural enemies present
Also present in large numbers at the Lower Coastal Plain Research Station yesterday were large number of red-tailed wasps (Campoletis sonorensis) in search of young tobacco budworm larvae to parasitize. The wasps were so common that it was almost easier to follow them from plant to plant than to search for the newly hatched larvae in buds. The large numbers of parasitic wasps also suggests that many of the larvae we saw this week may not make it to adulthood.

For those growers whose fields have tobacco budworm populations at greater than 10% infestation in small plants, have not observed parasitic wasps, and plan to treat, there are several insecticides available (see the NC Agricultural Chemicals Manual for specific recommendations). Some NC growers have used Coragen (a newer insecticide from DuPont) as a transplant water treatment for preventative control of tobacco budworm. Because there is relatively limited data against tobacco budworm using this application method of Coragen, I cannot say how effective or long lasting its activity may be. It also important to note the size of tobacco budworm larvae feeding on transplant water Coragen treatments. First instar larvae may not have fed on enough tissue to kill them, so growers should hold off on making foliar treatments until 2nd instar larvae are present. Caterpillars eat the vast majority of the leaf tissue they will consume as large larvae, so the injury caused by a 1st instar as it develops to a 2nd will not be significant. However, if growers who used Coragen in the transplant water for tobacco budworm control have 2nd instar larval populations above a 10% infestation level and need to apply a foliar, rescue treatment, they should select a different mode of action material, such as Tracer/Blackhawk (spinosad), Denim (emamectin benzoate), Orthene (acephate), or methomyl (Lannate). Growers who have used Coragen in the transplant water should not apply Belt or Coragen as their first foliar treatment following transplant.

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