Crop Scene Investigation - 21: What's the pest plaguing Jim's corn field?

Thursday, December 3, 2009

by BERNARD TOBIN

Corn seed treatments have become an important management tool for farmers, helping protect against insects that feast on fledgling plants.

When Tracey Baute, an entomologist with the Ontario Ministry of Agriculture, Food and Rural Affairs, visits fields in the spring, she's always on the lookout for insect populations that, in extreme cases, can overwhelm seed treatments. Often, the signs of insect damage are obvious, but the prime suspect is not always the guilty party, as Baute discovered last May when she visited Jim, a grower in Chatham-Kent. Here's how the clues unfolded at Baute's crop scene investigation.

"I visited Jim's farm at the end of May after getting a call from a seed company representative. When I arrived at the field, you could see square-foot gaps across the cornfield with missing plants. And the plants on the edges of the gaps were starting to die off.

"I reviewed the field history with Jim. He had planted a YieldGard corn borer hybrid mid-April in the sandy loam soil, which was in soybeans the previous year. The weather after planting was cold and wet and when I arrived the plants ranged from two to three leaves depending on the area of the field.

"Jim also told me that the farm suffered tremendous wireworm pressure and he was fearful that the wireworm had simply overpowered the seed treatment. In addition, he had limited success with his fall burndown, which meant that surviving weeds would act as a host for other insects such as black cutworm. This pest could be responsible for the damage I was seeing.

"After reviewing Jim's field history, I ruled out soil-borne diseases. His fertility and herbicide program also checked out. The answer to this mystery had to be in the soil. As I started digging around in the bare patches, I noticed no evidence of black cutworm leaf feeding damage on the plants and had no sightings of the actual insect. There was evidence of wireworm, but the population wasn't high enough to overwhelm a seed treatment.

"I then pulled a couple of the wilting plants on the periphery of the bare patches. These plants were obviously under stress and could contain the clue I was looking for. When I pulled a plant, I found a swollen seed with long, tiny pests feeding on it. These were definitely not wireworms. I immediately recognized them from their hard skeleton and mass of tiny legs, but I had never seen them attack a corn seedling like this."

"I would need to do a bait test to determine if these typically beneficial pests were indeed the culprits. If they were the guilty party, I knew why the insecticide was not protecting Jim's corn field."

Can you identify the pest in Jim's corn field? Send your solution to Better Farming at: rirwin@betterfarming.com or by fax to: 613-678-5993.

Correct answers will be pooled and one winner will be drawn for a chance to win a Wireless Weather Station. The correct answer, along with the reasoning followed to reach it, will appear in the next issue of Better Farming. BF