No. 22 Article 1/September 7, 2007

Larger Colony Densities of Corn Leaf Aphids Reported on Some Bt Corn Hybrids

An interesting journal article was recently published that serves to remind us that unintended consequences can result from our pest management decisions. By conducting a carefully controlled laboratory investigation, European scientists were able to make the following observations:

"Here we show a remarkable positive effect of Bt maize on the performance of the corn leaf aphid Rhopalosiphum maidis, which in turn enhanced the performance of parasitic wasps that feed on aphid honeydew. Within five out of six pairs that were evaluated, transgenic maize lines were significantly more susceptible to aphids than their near-isogenic equivalents, with the remaining pair being equally susceptible. The aphids feed on the phloem sieve element content and analyses of this sap in selected maize lines revealed marginally, but significantly higher amino acid levels in Bt maize, which might partially explain the observed increased aphid performance. Larger colony densities of aphids on Bt plants resulted in an increased production of honeydew that can be used as food by beneficial insects."

The plants used in this experiment were grown in a climate chamber at 27C with 60% relative humidity. The six Bt hybrids and their corresponding near-isogenic lines were the following: Bt11 (N4640Bt/N4640), MON 810 (MEB 307Bt/Monumental, TXP138/EXP138, Novelis/Nobilis), and Event 176 (Valmont/Prelude, Navaris/Antaris).

Aphids were placed on plants using clip cages to estimate colony performance. Corn leaf aphids (50 adults and 50 nymphs) were contained in clip cages on the sixth leaf of plants. After three days had elapsed, the clip cages were taken off and the plants were totally enclosed in tight mesh cages. Five weeks later, the number of aphids per plant was estimated.

For additional details concerning this research, funded by the Swiss National Science Foundation, please read the following paper: C.A. Faria, F.L. Wäckers, J. Pritchard, D.A. Barrett, and T.C. Turlings. 2007. High susceptibility of Bt maize to aphids enhances performance of parasitoids of lepidopteran pests. PLoS ONE 2(7): e600.doi:10.1371/journal.pone.0000600.

This summer there were numerous reports concerning large densities of corn leaf aphids in some fields. Considerable speculation ensued regarding potential explanations. We've reported previously that the approximately 40% of corn acres in Illinois planted to "stacked" Bt hybrids this season had doubled as compared with 2006 (19%). Is there a connection between increased Bt usage and large corn leaf aphid densities in 2007? Providing a direct answer to this question and many others will likely remain elusive under large-scale commercial conditions. The publication of this paper reminds us how much we have yet to learn about the potential ecological consequences of these powerful transgenic pest management tools.--Mike Gray

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