Knowing Your Roots

In our last blog post on the New Family Farm, Natalie Hernandez explained that there are many different aphid species that can pose a major threat to seed potato farmers as carriers of Potato virus Y (PVY) – a serious concern for seed producers.  In this follow up story, we will see how one of those aphid species—the soybean aphid, an accidentally-introduced pest from the Far East—is linked to two other deliberately-introduced invasive species, common buckthorn and the Asian lady beetle, which have each emerged as serious pests in their own right.

Blog 11_1It all began with common buckthorn, a fast-growing shrub that was brought to the US from Europe in the early 1900s to use as a windbreak plant to reduce soil erosion in Midwestern farm fields. It did a poor job as a windbreak but survived and flourished in southern Wisconsin and Michigan where it has become a serious threat to woodlots because it outcompetes natural vegetation.  

The next actor in this drama is the Asian lady beetle, that foul-smelling house invader that has been plaguing rural households in Wisconsin for close to a decade. The Asian lady beetle turns out to be a valuable predator of aphids on soybean. However, it was deliberately imported to the US in 1989 from China and initially released in Georgia to biologically control pecan aphids, which turned out to not be very successful in the pecan orchards.  However, the lady beetle then appeared in Louisiana and there was a beneficial general predator of aphids on a number of crops.  Eventually by 1995, the Asian lady beetle popped-up in Wisconsin.

Blog 11_2The 3rd actor in this saga, the soybean aphid itself, which ties the participants together, entered the picture in the late 1990s as an accidental introduction into northern Illinois (there is speculation that it was brought in unknowingly by tourists from Japan on edamame soybean plants – a Blog 11_3popular souvenir, that were discarded in the O’Hare vicinity). In the Midwest, the soybean aphid had arrived in “aphid heaven” with unlimited food in the millions of acres of soybeans, and no predators to hold it in check. All it lacked to wreak havoc was a way to survive the winter in our harsh temperate climate. Most aphids do this by adopting a sturdier winter host, usually a shrub, where they lay eggs and await spring.  This is where Common Buckthorn steps up to the plate; closely related to the soybean aphid’s natural winter host in China, common buckthorn was an ideal substitute.  The aphid was now able to become a year-round resident, leaving buckthorn in the spring to invade soybean (its favorite host) and multiply unabated.  And multiply it did!

Reaching massive populations and resulting in millions of acres of soybeans receiving insecticidal treatments. The late summer dispersal flights of winged forms were so large that they even interrupted our national pastime.  A baseball game in Toronto was halted when players (Cal Ripkin among them!) were unable to see due to these aphid pests.  The agricultural consequences were severe as clouds of winged aphids moved between fields and carried viral diseases to crops as diverse as green beans, peppers, cucumbers, melons, and to complete the circle, seed potatoes.

Blog 11_4This story of misadventure is not quite complete, however; remember the Asian lady beetle, which was innocuously making its way north looking for aphids to devour.  It now discovered unlimited supplies of food in the millions of acres of soybeans covered with soybean aphids—its favorite comfort food from home (China). The result was a huge beetle population explosion, and as these adults left the soybean fields in search of protected places to gather together for the winter, the south facing cliffs they used in China were nowhere to be seen. The alternatives were the rural barns and houses of Wisconsin where they accumulated by the bucketful, seeking every chink to invade our homes and becoming a major new household pest!

The lesson to be learned? Be careful what you import, deliberately or accidentally. US agriculture is plagued by dozens of pests that have made their own way to our shores. The soybean aphid story is an excellent example: 3 unrelated species, 2 considered to be beneficial, brought here deliberately and accidentally that have combined to create 3 major pest threats in 3 separate environments! For more information contact: Natalie Hernandez (nmhernandez13@gmail.com) or Dr. Russ Groves (groves@entomology.wisc.edu) or Dr. David Hogg (dhogg@cals.wisc.edu).

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