Crop Scene Investigation - 53: Sucking the life out of Sebastian's soybeans

Friday, December 5, 2014

by BERNARD TOBIN

It was a hot evening in early August 2012 when Sebastian rolled into a 50-acre soybean field with his RoGator self-propelled sprayer. After several days of plus 30 C temperatures, the Brant County farmer was about to lower the boom on an infestation of plant-sucking spider mites that were sure to steal yield.

Sebastian's insecticide included the active ingredient dimethoate, which works systematically in the plant. The soybean plant absorbs it and mites sucking on the plants usually die quickly. Excellent control can be achieved 24 hours after spraying. With a job well done, Sebastian called it a day just before dusk.

The farmer returned to scout the field 48 hours later and was surprised to see the mites continuing to feast on his soybean crop. Sebastian had invested $10 an acre to spray the crop and he was getting poor control – about 50 per cent of the mites were still feeding on the plants.

Sebastian quickly called his retailer to get some answers. Both he and his agronomist had agreed that the pests were indeed spider mites, so he reasoned that there had to be something wrong with the insecticide. Or could it be that the mites had somehow become resistant to the product?

The puzzle quickly landed on the desk of Willy Gubbels, United Agri Products  Canada's Eastern Sales Manager. "We checked with other retailers across Ontario and there were no reports of reduced efficacy or poor performance. We even checked the lot numbers and there was no evidence of any issues," explains Gubbels.

The next step was to check out Sebastian's sprayer and spray mix. After talking with the farmer, Gubbels was satisfied he had followed the label instructions, using the right rate, water volumes and pressure. The sprayer was also inspected and everything checked out, including the nozzles.

Gubbels then considered application conditions. Sebastian said he started spraying at around 7 p.m. The wind was less than five kilometres per hour and the mercury had dipped to 23 C after topping out at 30 C during the day. Sebastian knew that best practices required him to wait for the temperature to drop before spraying or efficacy could be reduced. Gubbels concurred. The farmer had done a good job.

Finally, Gubbels decided it was time to test the source and quality of the water Sebastian used in his sprayer. When the tests came back Gubbels immediately noticed that the pH level of the water samples ranged from 8.5 to 9.2.

He then asked Sebastian how much time elapsed between mixing the insecticide in the tank and when he actually started spraying his soybeans.

Sebastian thought about the question for a minute. After mixing the spray, he parked the RoGator, had supper and then headed to the field. He estimated it took him about two hours. The mystery was solved. Gubbels knew exactly what had happened.

Do you know why the insecticide failed to control the spider mites in Sebastian's soybeans? Send your solution to Better Farming at: rirwin@betterfarming.com or by fax to: 613-678-5993. Be sure to include your contact information.

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