No. 23 Article 5/October 8, 2004

Does the 2004 Fall Flight of Soybean Aphids Help Us Forecast Their Potential for 2005?

We are completing our fourth full season of soybean aphids, and with each year completed we learn a little more about this invasive species. Thus far, after the wake-up call we received in 2000, soybean aphid populations have been very cyclic in Illinois--large densities in 2001, low densities in 2002, very high densities in 2003, very low densities in 2004. So what's in store for 2005?

We have learned from David Voegtlin, entomologist at the Illinois Natural History Survey, that the fall flight of soybean aphids may provide some insight for the potential for soybean aphids for the next year. In the fall, soybean aphids leave their secondary host (soybeans) to fly to their primary host (buckthorn), where females mate with males to produce the eggs that overwinter. By sampling the population of flying soybean aphids, we obtain snapshots of their relative densities from one year to the next.

Nine suction traps are in place in Illinois to sample populations of flying soybean aphids. The rationale for and description of the traps are explained here.

The locations and operators of the traps also are provided. In the fall of 2002, Voegtlin observed a noticeable difference from 2001 in soybean aphid captures. In 2001, the suction traps captured virtually no soybean aphids in September. In 2002, the numbers of soybean aphids captured in September and October were noteworthy in suction traps located near DeKalb, Monmouth, Urbana, and Perry. The numbers of soybean aphids captured in suction traps in the fall of 2003 again were very low. So in the two years following captures of low numbers of soybean aphids in the fall, significant infestations of soybean aphids were few and far between. In the one year (2003) following captures of relatively large numbers of soybean aphids in the fall (2002), an outbreak of soybean aphids occurred.

Will this pattern repeat itself over time? It's too early to know for certain, but captures of noticeable numbers of soybean aphids in suction traps in the fall could very well portend significant infestations of soybean aphids the following year. This is especially true when populations of multicolored Asian lady beetles fluctuate in concert with populations of soybean aphids. In the falls of 2001 and 2003, numbers of multicolored Asian lady beetles, the most significant predator of soybean aphids, were very high--high enough to suppress soybean aphid populations the following year. In the falls of 2002 and 2004, numbers of these lady beetles were very low. The relative absence of multicolored Asian lady beetles in the spring and early summer of 2003 enabled soybean aphid populations to thrive. Will the same hold true for 2005?

To compare the numbers of soybean aphids captured in suction traps in 2001, 2002, 2003, and 2004, select the years of interest next to "See Trap Counts" at the aforementioned Web site. The charts displayed show the numbers of soybean aphids captured at each location during several weeks throughout each year.

So what do the numbers of soybean aphids captured in suction traps mean? Some words from Voegtlin about the captures during the week ending September 17 may be helpful: "About 1/3 to 1/2 of the catch is males. This migration is at least two weeks ahead of the 2002 flight and already we have about the same numbers that were collected that fall. There are . . . at least four more weeks of migration to go.

"I am not sure why the fall migrants are so early. In previous years, males were not collected until October. Perhaps [because] we have had a cool summer with some nights at or below 50 degrees, which is cool for late August and early September. I know that there are plenty of aphids on soybeans in far southern Illinois, but it may be that the temperature there has been high enough to prevent the population from developing these fall migrants. We know that when the photoperiod begins to drop below 14 hours, the development of these migrants will start; however, the response is often limited by higher temperature. I don't believe there are any data to tell us how warm it must be to slow this process or how cool it must be to trigger it."

At the time I wrote this article, the captures of soybean aphids for the weeks ending September 24 and October 1 had not been posted. However, Voegtlin has told us that the numbers of soybean aphids declined a bit during the week ending September 24. On the other hand, he also noted, "This is the first time since the traps have been in operation that we have seen fall migrants in every trap. In other words, fall migration is occurring throughout Illinois."

The stage is being set for 2005. Based upon a very limited amount of data over time, the potential for significant infestations of soybean aphids in 2005 exists. However, please understand that I am emphasizing the word "potential." Much can change between now and the soybean-growing season in 2005. In addition, higher temperatures in 2005 than we experienced in 2003 could significantly retard development of soybean aphid populations. Therefore, as is always the case with fluctuating insect populations, early and regular scouting will be the key to detecting infestations of soybean aphids in 2005.--Kevin Steffey

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