Friday, June 29, 2012

A Sharp Dressed Pest - Cucumber Beetles


(By U of I Plant Clinic Student Worker, Sean Mullahy)

These infamous garden pests come in two patterns, spotted and striped, and are fantastic at destroying all things cucurbit. So beware their interesting fashion sensibilities, and protect your garden against these nasty little pests. 

The first step to keeping them under control is proper identification. The spotted beetle is about ¼ inch in length and has yellowish green forewings with twelve distinct black spots. The striped cucumber beetles are nearly the same in size, also averaging about ¼ inch in length. These have much simpler fashion sensibilities, with two distinct black stripes running down each of their wing covers. The innermost stripe on each matches up with the one on its partner wing, making it appear as though they only have three black stripes. It can also be easy to confuse the striped cucumber beetle with the western corn rootworm. They have similar color and patterns, but the western corn rootworm has a yellow abdomen, and the striped cucumber beetle has a black abdomen. The larvae for both are striped, but spotted cucumber beetle larvae are whitish – yellow in color. Cucumber beetle larvae are much bigger than their adult stage, ranging from ½ to ¾ inches in length, and they have dark brown heads and three pairs of very short legs. 
Adult southern corn rootworm (a.k.a. spotted cucumber beetle), Diabrotica undecimpunctata howardi (R.L. Croissant, Bugwood.org).

Striped cucumber beetle (Whitney Cranshaw, Colorado State University, Bugwood.org)
Striped cucumber beetle (David Cappaert, Michigan State University, Bugwood.org)
Western corn rootworm beetle (Whitney Cranshaw, Colorado State University, Bugwood.org)

As for life cycle, both spotted and striped cucumber beetles overwinter as adults. The striped beetle is the only one that will be a big issue in early spring however, the spotted beetle has a much lower success rate in surviving the Illinois winter. It, however, can be a bigger problem in southern Illinois. Once active, they begin feeding, and can survive off the pollen and petals of many different plant species before cucurbits are planted or growing. Once the cucurbits are actively growing, they move onto these, with the ability to feed on foliage and fruits and truly devastating anything they can munch. These adults eventually lay eggs at the base of their hosts, and once hatched, the larvae will feed on the roots. Depending on location, this cycle can be repeated up to two times per season, with this only happening in warmer locales. 

If the direct damage to your tasty melons, butternuts, cucumbers, and all your other favorite hard shelled veggies wasn’t enough, the cucumber beetle also spreads a number of nasty pathogens. Throughout localized plantings, they will spread squash mosaic virus, which can be quite nasty to the fruits. They can also spread bacterial wilt of cucurbits, which affects most of your favorite fruit bearing cucurbits. 

Management of the Cucumber Beetle can be done preemptively with the application of a systemic insecticide to the soil at the time of planting. This however might be a bit much for the homegrowers out there. The only other option available is to spray a foliar insecticide when adult beetles show up.  Applications are recommended based on total adult beetle count, and if it exceeds one per ten plants at the seedling stage, or one per plant at flowering.  If you’d like to practice non-chemical control of this pest, the easiest thing to do is dress your plants in a mesh cloth to keep the pests out. The earlier the better for this method, and make sure the mesh is open enough to let in ample sunlight, while also being tight enough to keep the pests out.

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