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Soybean aphid population on the increase Tuesday, January 30, 2007
Agri News staff writer
FERGUS FALLS, Minn. -- Soybean aphid populations increased during the 2006 growing season.
University of Minnesota entomologist Ian MacRae found the earliest infestations near Euclid, a small town northwest of Crookston, on June 27. If the mild winter continues with warm winter temperatures and snow cover, aphids can overwinter and aphid numbers will remain high, he said at last week's Soybean Production Conference in Fergus Falls.
Aphids are born pregnant, MacRae said.
If infestation thresholds are reached early in the growing season, producers may have to make earlier treatment decisions, he said.
There is no detectable yield increase when producers treat low aphid populations, he said.
Producers shouldn't treat if a field stays at 250-aphids-per-plant and if those populations aren't doubling every seven days, he said.
"Let the natural enemies of aphids do their work," he said.
Aphid populations at 250 and then double every three to seven days, however, will cause yield loss.
Producers can use speed scouting to determine aphid numbers. Count the aphids on 11 plants. If the number of aphids reaches 40 per plant on six or fewer plants, don't treat and resample in seven to 10 days. If the number of plants is seven to 10, continue counting another five plants.
A speed scouting form is available on the web at www.soybeans. umn.edu. Follow the links to soybean aphid .
Producers should conduct two consecutive speed scouting samples, about five to seven days apart, before using an insecticide, he said.
MaCrae suggests farmer prioritize fields. Later-planted soybeans should be scouted first, he said. When soybeans reach the R6 stage, aphid populations have the potential to rebound in absence of natural enemies.
Natural enemies can be effective in aphid control. Predators such as ladybeetles, lacewings plus parasites and fungal diseases are effective. Multi-colored Asian ladybeetles help control aphids, he said.
The population of two-spotted spider mites may increase. Spider mites are often seen in dry conditions.
The last time spider mites reached infestation levels was in the 1988-89 drought.
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