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Better Moisture, Better Stem Weevil Tolerance?

Friday, April 15, 2005
filed under: Insects

Plant water balance is emerging as a critical factor determining sunflower tolerance to stem weevil larvae, according to JP Michaud, entomologist at the Kansas State University Research Center, Hays.

There is evidence to suggest that plant resistance to stem weevil larval development is reduced under drought conditions, so stem weevil damage may be more of a concern under dryland production. As well, KSU research indicates that sunflower grown under irrigation or limited irrigation has better tolerance to stem weevil larval development.

“We have observed that the addition of 5 inches of irrigation water around the time of flower bud initiation can be enough to reduce stem weevil larval survival in the plants by almost 50% compared to those that are drought-stressed in a dryland situation,” Michaud says.

He notes further that the abundant natural rainfall received at plot trial evaluation sites during the 2004 growing season reduced the numbers of stem weevil survival per plant to only 25% of what it was the previous year in the same variety under drought conditions. Adult emergence in spring was similar both years.

Michaud and other crop scientists at KSU are evaluating how sunflower development and yield are affected by stem-boring insects, primarily the spotted stem weevil and the longhorned beetle, also called Dectes or soybean stem borer. The research is funded in part through a grant by the National Sunflower Association and the Kansas Sunflower Commission.

Although both of these insects emerge around the same time in late spring, observations indicate that the stem weevil attacks the crop several weeks earlier, early to mid June, and during a much narrower window. Thus, later planting dates may reduce stem weevil damage, but will not affect the crop’s susceptibility to longhorned beetles that emerge over an extended period and attack the crop well into late summer.

Nevertheless, yield losses caused by stem weevils may be much greater because of the numbers of larvae that can occur in one plant, the nature of their feeding in the plant, and their role as the primary vector of Phoma black stem disease, that can further increase the chances of late-season plant lodging.

Carefully timed insecticide applications were used to generate plants treated and untreated for longhorned beetles, which revealed that the beetles had to direct effect on yield. However, since growing conditions were close to ideal last year, another year of evaluation is needed, Michaud says, since it is quite possible that yield losses directly attributed to the longhorned beetle could arise only when plants are stressed by drought or other factors.

In fact, longhorned beetle activity in sunflower may exacerbate damage caused by stem weevils. “We found that the more (longhorned beetle) larvae bore into a sunflower plant, the more stem weevil larvae are found in the plant toward season’s end. Apparently, the long-horned borers negatively impact the plant’s ability to resist the developing larvae of other insects, even if they have no direct effect on yield themselves,” Michaud explains.

See more information about Michaud’s research online at Click on the “Research” link, then “Research Forum Papers.” Also see the K-State Western Kansas ARC web site:

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