Spotted cucumber beetles (Diabrotica undecimpunctata) are very visible in prairies around here right now. Farmers know these creatures as the southern corn rootworm and they can be a pretty serious pest in crop fields. In gardens, they feed cucurbits (cucumbers, squash, melons), as well as other plants. The beetles do damage by chewing on the plants, but do more as a carrier of bacterial wilt when they carry bacteria from infected plants to uninfected ones. Larvae feed on roots while adults feed on leaves, stems and pollen.
Despite their status as a pest insect, spotted cucumber beetles are native insects that have happened to adapt well to the abundance of crops we’ve provided for them. When I as at our family prairie last weekend, I saw quite a few of them out in the grassland, feeding on flowers. I assumed that abundance was linked to the cornfields surrounding our prairie, but they were happily feeding on the pollen of curly cup gumweed (Grindelia squarrosa) and sunflowers (Helianthus sp), despite being within site of healthy corn plants.
My assumption that the beetles at our family prairie were there because of the surrounding crop land was challenged later in the week when I saw a bunch at the Niobrara Valley Preserve, which is pretty far removed from any corn fields. I guess I shouldn’t have been surprised to see them, given their mobility. The beetles migrate northward each summer into states where they can’t overwinter. With favorable winds, the beetles can move hundreds of miles per day. With that ability, finding their way to the Niobrara River Valley doesn’t seem like such a big deal.
I assume many predators take advantage of spotted cucumber beetle abundance each summer. I frequently see crab spiders feeding on them, and since the beetles aren’t very agile, they are probably easy prey for many other hunters as well. At the Niobrara Valley Preserve, I was surprised to see one being eaten by an antlion adult. I’ve written before about antlions, which are among the coolest predators in the world – at least as larvae. I’d forgotten, however, that some species can also be predatory as adults (most feed on pollen and nectar).
Most people don’t think of insects as long-distance travelers, but quite a few of them are. Strong fliers, including many butterflies, moths, and dragonflies, make long annual migrations. But even small clunky-looking beetles can move impressive distances each year, in search of food. That mobility will give them an advantage as growing conditions and plant communities continue to change rapidly in coming years. Less vagile (your ten dollar word for the day) animals will have to deal with those changes where they live, but others like cucumber beetles and many others will have the opportunity to fly to new areas. That should work out well for them – until they are caught by a crab spider or antlion…
The Prairie Ecologist 