Wednesday, December 10, 2014

Fall armyworm Resistant to B.t. Corn found in North Carolina: is your field next?

Doug Johnson, Extension Entomologist, University of Kentucky.

  Mr. Cam Kenimer, who runs our corn variety trials, sent me a link to an interesting article “Armyworms develop resistance to Bt corn.” The article which is actually about fall armyworm (FAW) reports that this pest carries a gene that makes it resistant to the B.t. trait Cry1F. This trait generally confers protection from caterpillar feeding in cotton (Widestrike™) and corn (Herculex 1™). This resistance has never before been reported further north than Louisiana or Florida

However, my colleague Dr. Dominic Reisig found FAW feeding and surviving on Cry1F, B.t. corn in North Carolina. While Bt resistance is a serious concern, the fall armyworm we face in Kentucky is from a different population than the ones causing problems for North Carolina growers. Nevertheless, Bt-resistant fall armyworms may become a problem for us in the long run so let’s take a closer look.

It pays to know a fundamental difference between the FAWs found in KY versus the ones in North Carolina. Fall armyworms can only survive the winter where its food plants grow all year round. Which means FAW cannot survive northern winters. Moths migrate northward each spring from two very different sources: southern Florida and southern Texas.

The Florida population contains individuals that have a gene modification providing resistance to the Cry1F trait. The Texas population does not seem to have individuals with this modified gene. Why only Florida? This is likely, because the Florida population interbreeds with populations in the Caribbean which are the probable source of the resistance. South Florida and the Caribbean have year-round FAW populations and lots of use and testing of single trait corn products. It appears that the Texas population does not mix with the Florida population very much so it has not acquired the resistance gene.

Fig. 1 General  FAW migration routes and time
of arrival in various parts of the U.S.
The Texas population migrates north and east, up the Mississippi river valley into Kentucky, Tennessee and the northern corn belt states. This Texas population does not carry the Cry1F resistance gene. The Florida population migrates out of south Florida moving along the Gulf (of Mexico) coast and northward along the southern and eastern side of the Appalachian Mountains. Though there does seem to be something of a mixing area in Louisiana and Alabama, the Florida population does not appear able to get further west and north on that route. It does however move up the east coast and eventually into New England and Canada. It look as if there may be some mixing of populations late in the season in eastern Pennsylvania or more northern states, but this is currently very unclear. The result of this separation in migration routes is that Kentucky is not exposed to the Florida, resistance gene carrying, FAWs.

This is very good for Kentucky, but no one can know how long it will continue. The Kentucky IPM program has been sending FAW moths to USDA scientists in Gainesville, FL for several years. These moths are tested to determine from which population they originate. Thus far it has always been the Texas population. Nevertheless, the mixing zone in Louisiana has to remain a worry. What is keeping these Florida population moths from moving further west and north? No one knows. But for now at least this is one problem that we don’t have.

Credit Figure 1. is an altered form of a graphic from: Sparks, A. 1979. A Review of the Biology of the Fall Armyworm. Fla. Entomol. 62(2):82-87.

References Armyworms develop resistance to Bt corn. Morning AgClips. from: Shipman, M. Armyworms develop resistance to Bt corn. Nov.18, 2014.

Fangneng H., J. Qureshi, R. Meagher, Jr., D. Resig, G. Head, D. Andow, X.Ni, D. Kerns, G.D. Buntin , Y. Niu, F. Yang, and V. Dangal.2014. Cry1F Resistance in Fall Armyworm Spodoptera frugiperda: Single Gene verses Pyramided Bt Maize. Plos One. DOI:10.1371/journal.prone.0112958.

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