LINDELL BEACH, B.C. – Recent studies have found that genetically modified squash plants resistant to viral diseases are more vulnerable to a fatal bacterial infection.
Researchers blame it on the feeding preferences of a tiny beetle.
In the mid-1990s, the U.S. Department of Agriculture approved GM squash that is resistant to three troublesome viral diseases: cucumber mosaic virus, watermelon mosaic virus and zucchini yellow mosaic virus.
Prompted by concerns that the GM varieties would cross pollinate with wild gourd populations, biology professor Andrew Stephenson of the Pennsylvania State University conducted a three-year study to investigate the inter-relationships between wild squash and GM squash in a natural field setting.
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His results, published last fall in the Proceedings of the National Academy of Sciences, focused on cucumber beetles, which threaten wild and cultivated squash.
The leaves and other organs of squash produce bitter compounds that are toxic to most herbivores, but the cucumber beetle uses them for defence and in mating. They tend to forage selectively on large healthy plants with many flowers.
However, the downside is that the beetle transmits Erwinia tracheiphila, which causes bacterial wilt disease when its contaminated fecal pellets fall on chewed leaves or inside the flower near the nectaries where the insects gather to mate. The bacterial disease is fatal to squash.
Stephenson naturally exposed wild and GM test plants to the zucchini yellow mosaic virus and the watermelon mosaic virus.
The wild squash became infected and a few GM plants showed mild signs of the viruses.
However, symptoms of bacterial wilt disease appeared in the test fields as each season progressed.
The incidence of the wilt disease on the GM plants spiked 17.5 percent compared to the wild plants at 10.9 percent.
Stephenson concluded that because the wild squash were stressed by the viral infections, the cucumber beetles selected the stronger, healthier, viral-resistant plants. In the process, they infected the GM plants with the Erwinia bacteria and increased their exposure to bacterial wilt disease.
