I have been getting more calls this week with people wanting to spray the aphids just to take care of them..even if they are not at threshold. Most realize that the natural enemies are going to get killed but they also assume 100% kill of the aphids and struggle to understand how the aphids can build back up so quickly in that same field.
The problem is, you don’t get 100% kill of the aphids. Even if you get 90% control, which is pretty good control, that leaves 10% of the aphids surviving on the plant. They are baby making machines and can build up their numbers back up to threshold in no time…especially now that you killed their natural enemies too, which are not baby making machines and take about 2-3 weeks to recover. Unfortunately the natural enemies need to find mates, produce eggs again, wait for about a week before hatching, then be larvae for another week or two (and hopefully the larvae feed on aphids) before finally becoming the hungry adults. Aphids are different. They are born pregnant and don’t need to mate. They pop out hungry nymphs that can suck on the plants immediately, just like the adults.
Here is a simplistic example to help get this point across. Let’s say a field is sprayed when there is only 100 aphids per plant. If you got 90% kill, that would leave 10 aphids per plant left surviving, and not many natural enemies left to eat these 10 aphids per plant. Aphids can double their population in 1.5 to 2 days, depending on the temperature. So if you started at day 0 with 10 aphids per plant, Day 2 would have 20, Day 4 would have 40, Day 6 would have 80, Day 8 would have 160 and Day 10 would have 360 aphids per plant. You’d essentially be back up above threshold in less than two weeks time..not saving you any money or time for your effort in spraying too early.
At least wait until you are confident that the aphid population is rising above 250 aphids per plant. That is truly the only way to know whether the natural enemies are not doing their job and that a spray is finally necessary.
Spraying before the threshold, grinds all of your biological control to a complete halt, when they may have actually been the ones making a difference. Otherwise, wouldn’t every single soybean field in Ontario need to be sprayed every single year?