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Vol. 19, No. 2, July 22, 2004 In
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Are soybean aphids regulated by weather, natural enemies or both? |
Since its initial detection in the North Central U.S. in 2000, the soybean aphid has become a key pest of concern in soybean. In 2003, over 42 million acres of soybean in the North Central U.S. were infested and more than seven million acres were treated with insecticides to control the soybean aphid. While 2004 seems to be shaping up as a relatively low aphid year so far, we are just now entering the time when outbreaks typically occur. Many factors contribute to determining if soybean aphids reach outbreak levels or not. Among these are weather and the impact of natural enemies. In this article we focus on the interaction of weather and natural enemies in suppressing soybean aphid. There has been a longstanding debate among entomologists as to what factors have the greatest influence on pest populations. One school of thought is that weather is the main factor and all other influences play a minor role. Another school of thought contends that competition and the impact of natural enemies are the dominant factors regulating pest populations. As in most such debates the truth is probably somewhere in between. In the case of soybean aphid, a picture is beginning to emerge that suggests that both weather and natural enemies interact to regulate aphid outbreaks. Cool weather conditions favor soybean aphid outbreaks in several ways. Recent studies in Minnesota show that the soybean aphid reaches its maximum reproductive potential when temperatures are around 81 degrees and that even short exposures to temperatures over 90 degrees greatly decrease aphid reproduction. Coupled with this is the fact that many natural enemies are less active at cooler temperatures. For example, in 2002 many days exceeding 90 degrees and almost no aphid outbreaks were reported throughout the region. In contrast, 2003 was more moderate and aphid outbreaks were extensive. Observations of this sort tend to support the idea that weather is the driving factor. On the other hand, studies in Michigan and Indiana have documented 22 species of predators in soybean fields that readily feed on soybean aphid. Of these, lady beetles, minute pirate bugs, hoverflies, lacewings, carabid beetles and spiders are common in most fields. Parasitic wasps that are either native or previously introduced are just beginning to show up in some surveys but at present contribute little to soybean aphid biocontrol. Finally, certain predaceous aphid flies and midges lay eggs in the colonies and their larvae help reduce aphid numbers. Thresholds developed for soybean aphid generally suggest that action should be taken once aphids reach 250 per plant. All of these studies have been conducted in open fields with natural enemies present. Another way to think about this is that once aphids reach about 250 per plant, natural enemies are unlikely to stop them before they reach outbreak proportions. Thus, it is important that natural enemies act early to suppress soybean aphid populations rather than just respond to outbreaks. While it's pretty hard to experimentally change the weather, we can alter natural enemy numbers by use of exclusion cages. In this way we can ask the question, given the weather conditions in a particular year, are natural enemies an important factor in suppressing soybean aphid? Our experiment consists of establishing 1-meter square plots in soybean fields and infesting them with 10-11 aphids per plant representing a typical early season infestation. We then cage one-half of the plots and leave the other half un-caged (fake cages that allow natural enemies to enter are always included to control for possible cage effects but for simplicity are not discussed here). Pairs of caged and un-caged plots are repeated 6-10 times throughout the field and aphid populations are counted weekly. If weather were the sole driving factor, we would expect to see nearly equal numbers of aphids in caged and un-caged plots over time. In 2002, aphid populations in open plots (i.e. exposed to natural enemies) never exceeded eight aphids per plant (See figure). However, when natural enemies were excluded, populations reached 160 aphids per plants by mid-July. Over the next two weeks these numbers reached over 2,000 aphids per plant in exclusion plots, while open plots never exceed 20 aphids per plant. Thus, natural enemies were critical to keeping populations below thresholds in 2002. In 2003, just one week without natural enemies resulted in rising aphid populations (7/21) and by two weeks (7/28) aphid populations in exclusion plots were out of control (See figure). However, also notice that in the open plots populations had reached about 200 aphids per plant on 7/28 (i.e. approaching the 250 aphid/plant threshold). We all remember that 2003 was a very bad year for aphid infestations. Thus, while natural enemies suppressed soybean aphid populations and delayed outbreaks, nonetheless, outbreaks did occur in many fields. In 2004, our data again indicate that natural enemies are very effective in suppressing aphid populations (See figure). Last week's counts (7/6) showed over 300 aphids per plant in exclusion plots and less than 10 per plant in open plots. It remains to be seen if natural enemies will keep overall populations below thresholds as in 2002 or if conditions will once again favor aphid outbreaks as in 2003. In production fields, aphids numbers across Michigan and in surrounding states continue to be very low. The percentage of infested plants in the fields monitored by the MSU Field Crops Entomology lab is less than 5%, and often less than 1%. In other words, the vast majority of plants are still aphid-free, while only a few plants have one or several aphids. In comparison, aphid populations were already building in early July 2003, and by late July, fields were being treated. Even in 2002, the last "low" aphid year, most plants had aphids by the end of July, although in low numbers. Thus, each year from 2000 to 2004 has been a bit different! In conclusion, it's both! Weather conditions appear to set the basic trajectory for soybean aphid population growth. Temperatures around 81 degrees favor maximum aphid reproduction, while temperatures over 90 degrees slow population growth. In 2003, aphid populations were growing slowly due to unfavorably hot conditions and natural enemies had no trouble suppressing their populations throughout the season. In contrast, in 2003 more moderate conditions favored the soybean aphid and while natural enemies suppressed and delayed aphid outbreaks for several weeks, they were unable to prevent the eventual build-up of heavy late season populations. Looking at all three years together it is clear that in the absence of natural enemies, soybean aphids reach outbreak levels regardless of the weather! Our current research is aimed at determining if we can enhance the suppressive effect or our existing predator-dominated communities by augmenting them with parasitoids. While growers cannot change the weather they can do things to encourage effective natural enemy populations. Most of the natural enemies mentioned above require food (pollen and nectar), alternate prey (aphids or other small insects) and shelter from adverse conditions. Managing habitats on your farm to promote natural enemies may be one way to help ensure that these helpers are always present. For more information on habitat management for natural enemies see the following resources Farmscaping to Enhance Biological Control. http://attra.ncat.org/attra-pub/farmscape.html 'Naturalize' Your Farming System: A Whole-Farm Approach to Managing Pests. http://www.sare.org/publications/farmpest/index.htm |
News update from Pesticide Education |
The following information is from the MSU Pesticide Safety Education Program's newsletter published by Christina DiFonzo and Carolyn Randall. Teens and Ag The latest on West Nile Virus
in Michigan The latest on emerald ash borer Buyer beware of the Love Bug!
The marketer, Prince Lionheart Inc, has been required by the FTC to prove its claims, which also include "protecting infants from West Nile Virus." Read more from the FTC at: http://www.ftc.gov/opa/2004/06/lionheart.htm. Incidentally, ultrasonic insect repellers in general do not stand up to scientific scrutiny. |
Regional reports |