Economic Benefits of Bt Maize
Maize containing a transgenically expressed toxin originating from
Bacillus thuringiensis
(Bt maize) is planted across the United States to combat insect herbivory. Non-Bt Maize is also planted alongside Bt maize fields to provide refuges for the insects, which helps to prevent resistance to Bt maize from evolving.
Hutchison
et al.
(p.
222
; see the Perspective by
Tabashnik
) analyzed how Bt maize affected the economic impact of the European corn borer moth in the midwestern United States, as well as its population dynamics. Larval density, a predictor of corn borer population size, has dropped in correlation with the percentage of Bt maize planted. In the highest Bt maize producing state, the positive effects of Bt maize in controlling insect herbivore populations extended to non-Bt maize. Furthermore, the decrease in insect populations demonstrated an overall economic benefit outweighing the overall extra costs associated with planting Bt maize.
ULV malathion (9.7 oz AI/acre) was applied by air to a 16 square-mile area during August of 1968, 1969, and 1970. Adult Diabrotica virgifera LeConte populations were reduced the following season by 39, 54, and 72%. No economic infestations occurred in the treated area the year following any application. Postspray migration of beetles was very limited, but adult migration during the peak emergence period the following season contributed to repopulation of the treated area. Migration and fecundity appear to be density-dependent factors which favor increases under low populations. Area suppression does not appear economically feasible, but adult control in individual fields may be an acceptable alternative to soil insecticides applied for larval control. A model was developed for timing treatments against adults; treatments between Aug. 1-15 should result in adequate population suppression to prevent damage the following season. Mid-August population levels of 1.0 beetle/ plant were an acceptable economic threshold for determining the need for control measures. The western corn rootworm, Diabrotica virgifera LeConte, is presently controlled by prophylactic application of soil insecticides at planting time; while preventing larval damage, those treatments may not greatly reduce emerging adult populations (Pruess et al. 1968). Few alternative control methods have been investigated (Hill et al. 1948, Calkins et al. 1970) and suppression for longer than 1 year has not been accomplished by a single treatment or method. The study reported here was conducted to determine the population suppression possible by adult corn root worm control within a large area prior to oviposition.
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