FAQ: The Illinois Aphid Swarm

A student at the University of Illinois navigates an aphid swarm between classes.

A student at the University of Illinois navigates an aphid swarm between classes.

We've had plenty of traffic here at the Myrmecos Blog as bewildered midwesterners look for answers about the swarm of tiny insects that has descended on our cities this week.  As best as we can tell, here's the scoop.

Q: What are the annoying little bugs that are swarming Central Illinois this week?

A: They are soybean aphids (Aphis glycines).  These small insects feed in summer on soybeans, overwinter as eggs on buckthorn (Rhamnus spp.), and feed in spring on Buckthorn before flying back to soy.

A soybean aphid

A soybean aphid up close

Q: What are they doing?

Summer has ended, and the aphids are leaving the soybean fields to look for buckthorn to lay their eggs on.

Q: Why are there so many of them?

We don't really know.  Part of the answer is that Illinois farmers grow a lot of soy, so there are plenty of resources to support a bumper crop of bugs.  Also, soybean aphids are a new pest for our continent, arriving here from Asia less than 10 years ago.  They might have escaped their native diseases, parasites, and enemies.  It may also have taken them this much time to build up to the high numbers we're seeing this year.  The cooler, wetter weather earlier in the summer and lower numbers of ladybird beetle predators may also be a factor.

Q: Are soybean aphids dangerous?

No.  These aphids do not bite or sting, nor do they transmit any human diseases.  Their immediate negative effects are limited to their host plants.

Q: What can I do to get rid of them?

Unfortunately, there is little that can be done.  Pesticides are a greater health risk than the insects themselves, and given the sheer number of aphids any strategy short of mass spraying across the entire county will not have much effect.  We recommend waiting it out and spending more time indoors if the bugs bother you.

Insect repellents will not work either, as repellents target insects that are attracted to us.  The aphids are merely incidental.

Q: Will they harm my garden?

Not unless you have buckthorn or soy.  These aphids are picky when it comes to feeding on plants.

Q: When will they go away?

Their numbers should subside once cooler weather hits.  We hope.


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thank you for this q&a. they were freaking me out.

Thanks Jen! They've been freaking *me* out, and I'm an entomologist.

What are they doing? They are leaving soybean as it dries down and going to buckthorn to mate (they will meet males there for the first time--all the ones on soybean are females) and then lay eggs that will overwinter until next spring. Dave Voegtlin reports that the buckthorn leaves are wall-to-wall aphids.

In 2001, swarms delayed a baseball game in Toronto http://ipm.illinois.edu/bulletin/pastpest/articles/200120b.html

By Gail Kampmeier (not verified) on 21 Sep 2009 #permalink

ya, i get hundreds of these stuck in my hair and shirt/shorts every time i bike to class which is a rather lengthy 2 miles of road + lots of shade.

These buggers seem to enjoy swarming in the shade.

Does this mean that eradicating buckthorn would be one way of tackling the aphids? (um, obviously mot this year).

Something similar was done for yellow rust - I should try to find the Danish PhD thesis that discussed it.

I hope there's gonna be NO ant tending to these creatures :) in the future. Is that a real possibility??? Or ants don't like soybeans that much :P

Great write-up on this, Alex. And that first photo tells the tale...

I read somewhere that this happened in 2003 or thereabouts. The aphids were to blame for the loss of about 25% of that year's soy crop. I'm guessing this kind of swarm is not so much cyclical as related to favorable weather during the year...?

No health concern, but what if aphids are accidentally ingested? ... What about accidentally inhaled?

As might be assumed such an extraordinary number of aphids in the air is the product of the past few months. For most of the summer there were few soybean aphids on the Illinois soybean crop. In mid-August this changed with migration into Illinois, most likely from Wisconsin, Minnesota and Iowa where high populations were found all summer. August temperatures were in the 70's and 80's and this is the range where the soybean aphid has its maximum reproductive rate. Field populations in Illinois exploded in August and early September. I visited many soybean fields to monitor the aphid and noted that in many fields there were few natural enemies, including the multicolored Asian lady beetle. In addition, this late in the season many of the fields were near maturity, and farmers made the decision to not spray these fields. So.... the soybean aphids were allowed to continue their population growth. In autumn when the photoperiod is less than 14 hours and temperatures are below 50 F the soybean aphids change their host preference. In late August we had night temperatures near 45 deg. F. Winged aphids produced on soybean after this combination will fly in search of Rhamnus spp., here primarily Rhamnus cathartica, european buckthorn. These conditions are two to three weeks earlier than normal and the large fall flight is about that far ahead of previous years. The migrants that arrive on buckthorn, feed and produce nymphs that mature into wingless oviparae. These oviparae remain on buckthorn and are mated by males that are produced on soybean and migrate to buckthorn. Overwintering eggs are laid at the base of the buds on buckthorn twigs,

Elimination of buckthorn would eliminate this aphid from N.A., however, buckthorn is very widely spread very abundant in some parts of the Midwest. In Illinois most of the buckthorn is found north of I-80. It is extremely abundant in Michigan, Wisconsin and Minnesota reaching densities of several thousand stems per acre. We have scattered buckthorn in Champaign, and other towns and cities in central Illinois. These are insignificant in yearly survival of the soybean aphid. Most years there is no successful overwintering in this part of Illinois. The aphids we get are summer migrants produced on soybeans north of us.

This in an interesting situation. The entire system is exotic. The soybean, aphid and primary predator are from Asia. The winter host is from Europe.

By David Voegtlin (not verified) on 22 Sep 2009 #permalink

While an undergrad, I worked at the UW Arboretum, and our marching orders were to remove all buckthorn, which was virtually impossible. It seems like most natural enemies for aphids move in after the population is quite large, so if the biggest populations were relatively late in summer, they probably temporally escaped most predators/parasitiods, too.

This calls for the introduction of an ineffective parasitoid that will have no effect on aphids or buckthorn, but will reassure the public that "something is being done," provide information for numerous press releases, and support the career of several dozen entomologists for at least a decade.

Alternatively, you could just pave the entire state.

By Joshua King (not verified) on 22 Sep 2009 #permalink


Invasive Asian beetles have been a much bigger problem over the past few years than these soybean aphids (which have been freaking me out as well this past week). These are the Asian lady beetles that bite, smell, and try to get into your house in huge numbers.

The Cornell group is working to learn about and promote native lady beetles. A leading theory in the decline of North American species is tied to the expansion in range of the Asian lady beetles. We don't need more of those... it's scary to think a boom in invasive 'gnats' might be followed by a boom in those things in a few weeks.


By Sheena Beaverson (not verified) on 22 Sep 2009 #permalink

Ah, one more wonderful consequence of the industrialization of our food supply. This time our fetish for monoculture probably won't sting us, but soon enough it will!

Ok, well then, it seems we need more of the native beetles. But, are soybeans a native crop to North America? If not, then we might expect that asian beetles and throngs of aphids would result. I totally agree with Ed.

How comes this year is so much worse than last year or any other previuos year?

Rosi, my guess is bc this year they stopped releasing the Asian beetles to fight the aphids bc the beetles were a nuisance too.

Thank you for the detailed explanation. I know you mentioned that these aphids aren't dangerous to humans, but are they dangerous to house pets like cats and dogs?

Anyone know what kind of buckthorns they prefer? Rhamnus frangula can real really high densities in disturbed riparian areas. It also probably doesn't help that it's a Eurasian exotic.

By Rob Clark (not verified) on 23 Sep 2009 #permalink

*reach really high

By Rob Clark (not verified) on 23 Sep 2009 #permalink