farmgate: Soybean Aphids: Have You Found Any Yet?
Most soybeans in the Cornbelt have been planted and have emerged, but the early soybeans that have blooms may also have minute visitors that seem to be problematic in odd-numbered years. Since this is 2009, it must be time for soybean aphids, and if you raising soybeans across the northern part of the Cornbelt, they may have made themselves right at home in your fields. So what do you do now?
Soybean aphids currently are being found on very young soybeans in Iowa, South Dakota, Wisconsin, Michigan, and Ohio, as well as in southern Canada. They may well be infesting fields in the northern tiers of Indiana and Illinois, as well as in the southern part of Minnesota, where they have been found in years past.
PLEASE NOTE: THIS DOES NOT MEAN YOU SHOULD START SPRAYING!
Thank you, now that your attention has returned to the page, we can focus on the problem and discuss steps for assessment and potential response.
Iowa State entomologist Erin Hodgson writes in the current issue of the Integrated Crop Management News that a colleague who has monitored early season activities of soybean aphids found that 70% of the soybeans in his northern Iowa research plots were infested with an average of 5.5 aphids per plant earlier in the week. The noteworthy aspect is the unusually early timing of their arrival, and the difficulty that entomologists are going to have in predicting the extent of the infestation and making response recommendations to soybean growers.
Your first line of defense are the multi-colored Asian lady beetles, and other insects that prey on soybean aphids. Hodgson says they are very good at finding aphids in low densities, which currently is the case. If you find a few aphids here and there during your soybean scouting, your immediate response to spray is the wrong response, since it will also destroy the beneficial insects that just may keep your problem under control without undue expense.
Your second line of defense is the heat and humidity, since that fosters growth of a particular fungus that infests aphids. Hodgson says the daytime humidity is right, but evenings are not cool enough for the fungus to produce spores and spread.
Your third line of defense is the possibility that aphid populations will rapidly increase and when they do, they grow wings and fly away, with the populations rapidly diminishing. While this is a strange event, it may mean the aphids may not reach proportions that will create yield damage.
Remember that the economic threshold is 250 soybean aphids per plant, and when that trigger point is reached it signals the time to arrange for a rescue spray. That population may not devastate soybean fields, but finding that many aphids per plant buys you enough time to spray as the aphid population rises to the real point of economic damage.
Hodgson says aphids do not do well in hot, dry conditions, which is one reason their impact area is generally across the northern part of the Cornbelt. He says daily temperature and humidity have to be optimal for aphid reproduction. He also says, “Fields that are sprayed before bloom may require a second insecticide application if aphids rebound or if the field is reinfested. It is important to keep scouting for aphids through seed set even if treated with an insecticide earlier that season.”
If you are scouting soybeans for aphids, Hodgson recommends scouting every field on a regular basis so you can get a good record of how fast the aphid populations are increasing. He says take a close look at 20 to 50 plants. If aphid numbers are minimal and not increasing, then you may be able to save money and increase your profitability.
Entomologist Tracy Baute of the Ontario Ministry of Agriculture suggests the more advanced a soybean field is in the growth stage, the more likely aphids have found them. But says aphids will be on the newest trifoliates starting to emerge.
Summary:
Soybeans in the northern part of the Cornbelt may be the target of soybean aphids, since notable aphid populations have appeared unusually early in the growing season. Entomologists are quick to recommend against immediately spraying them in fear of also killing predatory insects that could control the aphid population without the need for spray. Aphid populations can also be kept in check by fungus, as well as high temperatures.
Posted by Stu Ellis on June 25, 2009 12:16 AM to farmgate