Corn Earworm Infestation :(

I had great hopes for our sweet corn harvest this year.  I planted, diligently watered and weeded, and watched as the corn grew amazingly.  The plants looked healthy and happy.  Then, while visiting, my sister noticed that the silks were looking odd.  They were stunted or falling off.  They were shorter and drier than they should be given the time of year and size of the corn.  She thought the corn might have a worm problem.  I pulled back the husk on one to discover little worms feasting on the silks and moving down into the kernels.  As far as I know, this is corn earworm.  Every single plant was infested.  Forty plants ravaged.

Now, if you have a stronger stomach than I do, you don't worry about these kinds of things and you harvest, cut off any affected parts, wash off the worms, cook and munch away.  Truthfully, that's not me and they had worked quite deep into each cob, ruining kernels and defecating as they went.  So this year's crop was a bust.  

Doing a bit of research online, I read in an Iowa State University article published in August of this year that "many commercial and residential growers noticed unprecedented infestations and injury from corn earworm larvae this year."  So, apparently I'm not alone in this plight.

According to a University of Florida article on the corn earworm, they can be found throughout North America except for northern Canada and Alaska.  It is known to survive as far north as 40 degrees north latitude.  But there are overwintering and immigrant populations that can impact crops. In the relatively mild Pacific Northwest of the United States, corn earworm can overwinter as far north as southern Washington.  Apparently, the corn earworm moth doesn't just favor corn as a host but a variety of other vegetables from tomatoes to cucumbers and asparagus to pumpkins.  

"On corn, its most common host, young larvae tend to feed on silks initially, and interfere with pollination, but eventually they usually gain access to the kernels. They may feed only at the tip, or injury may extend half the length of the ear before larval development is completed. Such feeding also enhances development of plant pathogenic fungi. If the ears have not yet produced silk, larvae may burrow directly into the ear. They usually remain feeding within a single ear of corn, but occasionally abandon the feeding site and search for another."

"Although numerous natural enemies have been identified, they usually are not effective at causing high levels of earworm mortality or preventing crop injury.  Eggs are very difficult to detect, and larvae burrow down into the silks, out of the reach of insecticides, soon after hatching."

What Can You Do To Prevent or Treat Corn Earworm?

Insecticides… 

"Corn fields with more than 5% of the plants bearing new silk are susceptible to injury if moths are active. Insecticides are usually applied to foliage in a liquid formulation, with particular attention to the ear zone, because it is important to apply insecticide to the silk. Insecticide applications are often made at two to six day intervals, sometimes as frequently as daily in Florida. Because it is treated frequently, and over a wide geographic area, corn earworm has become resistant to many insecticides.  Mineral oil, applied to the corn silk soon after pollination, has insecticidal effects. Application of about 0.75 to 1.0 ml of oil five to seven days after silking can provide good control in the home garden."

Given their resistance to many insecticides and my desire to use little to no insecticides, if possible, at most I will try the mineral oil route on the corn silk right after pollination, as suggested.

Cultural practices

"Trap cropping is often suggested for this insect. Lima beans also are relatively attractive to moths, at least as compared to tomato. However, it is difficult to maintain attractant crops in an attractive stage for protracted periods. In southern areas where populations develop first on weed hosts and then disperse to crops, treatment of the weeds through mowing, herbicides, or application of insecticides can greatly ameliorate damage on nearby crops. In northern areas, it is sometimes possible to plant or harvest early enough to escape injury. Throughout the range of this insect, population densities are highest, and most damaging, late in the growing season. Tillage, especially in the autumn, can significantly reduce overwintering success of pupae in southern locations."

I grew pumpkins, another listed host plant, very close to the corn this year and the earworms still destroyed the corn crop.  I will see if tillage and changing the corn location next year helps to control things.

Biological control

"The bacterium Bacillus thuringiensis, and steinernematid nematodes provide some suppression. Entomopathogenic nematodes, which are available commercially, provide good suppression of developing larvae if they are applied to corn silk; this has application for home garden production of corn if not commercial production (Purcell et al. 1992). Soil surface and subsurface applications of nematodes also can affect earworm populations because larvae drop to the soil to pupate (Cabanillas and Raulston 1996). This approach may have application for commercial crop protection, but larvae must complete their development before they are killed, so some crop damage ensues."

If the mineral oil on the silks doesn't seem to help, I may see how hard it is to get some of these listed nematodes and apply to the corn silk.

Weedy Pete 

Sources

My backyard

Iowa State University Extension and Outreach

University of Florida Institute of Food and Agricultural Sciences