CARMAN, Man. — One soybean plant had hundreds of tiny green aphids sucking on it.
The other one had almost none.
The difference was that one was covered by a bag and protected from other insects, while the other was left unprotected in the field.
“There are quite a few predators that will feed on soybean aphid,” Jordan Bannerman, a University of Manitoba entomologist, said during a Manitoba Pulse and Soybean Association field tour at the Carman research farm.
Added fellow U of M entomologist Alejandro Costamagna: “You have lots (of soybean aphids) in here, and you have a hard time finding them there (in the unprotected plant), although they started at the same time with the same number of aphids.”
Bannerman, Costamagna and Manitoba Agriculture entomologist John Gavloski knew that the plants had an identical number of aphids on them two weeks before because they placed them on the soybeans.
The experiment and demonstration was an attempt to show farmers that there are good reasons to avoid applying insecticides before critical threshold levels are reached.
Natural predators such as ladybeetles, green lacewings and minute pirate bugs were able to slash aphid numbers to insignificant levels in this field even when the plants were deliberately infested by aphids, while a covered plant became significantly infested.
The beneficial insects move into fields from the edges, hedges, trees and surrounding crops, so even if aphids appear in a crop, farmers need to also look for the beneficial ones because they are probably there.
Farmers who hold back from using insecticide until threshold levels for economic damage are reached are giving predator bugs the chance to supply free control.
“They’ll find an aphid, impale it, hold it up, suck the juice out, put it down and find another,” said Gavloski about hover fly larvae.
Added Bannerman: “For blind maggots, they’re brutally efficient at eating aphids.”
Manitoba soybean growers should look for a threshold level of 250 bugs per plant “and rising” before considering spraying. The “and rising” element is important because the actual threshold for damage is 675 aphids per plant, but beginning to consider treatment while numbers are rising “gives you a week lead time,” said Bannerman.
“If you are at this threshold, you should most likely scout again in a couple of days.”
Costamagna said natural predators have a greater ability to offer control than many farmers realize if they are just looking at aphids.
“They always have this impressive potential to grow,” he said.