Wheat’s genetic resistance to Hessian flies has been failing, but a group of Purdue University and USDA scientists believe that other plants might soon be able to come to the rescue.
The Purdue and USDA research team developed a method to test toxins found in other plants on Hessian fly larvae. The test simulates the effect of a transgenic plant without the lengthy and costly procedures necessary to actually create those plants.
"For years, people have tried to develop a bioassay, but that hadn’t happened until now," says Richard Shukle, a research scientist with the USDA–Agricultural Research Service (ARS) Crop Production and Pest Control Research Unit working at Purdue. His findings were published in the Journal of Insect Physiology.
Shukle says the 33 genes known to give wheat resistance to Hessian fly attacks have been failing, causing scientists to develop methods to stack those genes together as a defense. But another solution could include adding other plants’ toxins to wheat to bolster its defenses.
The problem has been with the unique way that the Hessian fly larvae attack and feed off wheat. The larvae secrete a substance onto the plants that creates a sort of wound on the plant tissue, opening it up for the larvae to feed on.
New testing procedure. Toxins can be tested on other pests by adding them to a plant-based artificial diet and feeding them to the insects. But Hessian fly larvae wouldn’t take the bait, meaning that until now the only way to test poisons from other plants was to create lines of transgenic wheat for the flies to eat.
|Hessian fly larvae that have consumed snowdrop lectin with an artificial diet (left) are less developed than those given a control diet (right).
"This feeding assay is significant. This gives us a way to test these toxins," says Christie Williams, co-author of the findings and a USDA-ARS research scientist. "A preliminary chemical assay might give us promising results. But then you could go to all the trouble of making a transgenic plant based on that chemical test and have it not work."
To get the toxins into the fly larvae, the scientists allowed Hessian flies to lay eggs on the leaves of wheat seedlings. When the eggs hatched, the plants were taken from the soil, their roots cleaned and trimmed, and then replanted as hydroponics with the toxic proteins added to the plants’ water.
"The plant is acting like a big straw taking up the toxins," Williams says.
When the fly larvae attacked and fed as usual, they ingested the toxins that were taken up through the water.
Tests that use antibodies to detect the presence of a particular protein showed that the larvae ingested the toxins in the water. The team tested nine lectins—antinutrient proteins that disrupt digestive function. Hessian fly larvae responded in particular to snowdrop lectin, which comes from snowdrop bulbs, a flowering plant.
Larvae that ingested the snowdrop lectin developed only half as fast as the control larvae. There was also evidence of disruption of the microvilli, fingerlike extensions in the midgut that aid in nutrient uptake.
"It is possible that snowdrop lectin, by itself, could give wheat better resistance to the Hessian fly," Shukle says.
The scientists plan to have a transgenic version of the wheat developed for further testing.