Features Agronomy Soybeans
Aphid antibiosis rating gives growers new tool

Soybean aphids have quickly become the single largest insect pest Canadian soybean producers face since they were first discovered in Michigan fields in 2000. Researchers at Kansas State University, working in conjunction with Pioneer Hi-Bred, have discovered that soybean aphids reproduce more quickly on some soybean varieties than on others. Researchers are still trying to determine exactly why some varieties are able to inhibit aphid reproduction. In the meantime, Pioneer has given all their soybean varieties an aphid antibiosis rating to help producers make seed selections.

Scouts should consider thresholds and assess yield loss potential.

“No soybeans are resistant to soybean aphids,” explains Glen McDonald, Pioneer area agronomist at Mt. Brydges, Ontario. “Some varieties do have some mechanism in them that either favours, or doesn’t favour, aphid reproduction. We call this plant reaction aphid antibiosis. We’ve looked at our seed varieties and given those with better antibiosis properties an above average rating. Other varieties, with slightly less, are rated average. Varieties that allow aphids to thrive are given a below average rating.”

McDonald says that at present there are only small differences between varieties with an above average aphid antibiosis rating and those with an average rating, but there are definite differences. In some cases it might make the difference between whether they need to be sprayed.

“Planting a variety with an above average aphid antibiosis rating doesn’t mean you won’t need to spray if you have high aphid pressure in your area,” McDonald says. “It will probably hold them off for a certain period of time, but if it’s a high aphid infestation, then you may end up spraying them as well. However, if you’re in a lower population scenario, then you may not have to spray above average varieties. Some of our new lines show some pretty good tolerance.”

Until resistant lines are developed and reach the market, producers need to maintain their scouting to make sure that soybean aphids are not causing yield loss. If aphid numbers reach 250 aphids per plant, when the soybeans are in the R1 to R5 stage, then fields should be treated within seven days to prevent economic losses.

Aphids, with their black cornicles or tailpipes, can vary greatly in numbers in just a few days, which is why scouting on a regular basis is so important.

Aphids are small and light green in colour, with small black cornicles or ‘tailpipes’. They generally prefer new growth. Since aphids are not always evenly distributed throughout the field, it is wise to keep an eye open for hot pockets. McDonald recommends picking representative spots in the field and then monitoring them every few days depending on infestation levels.

They feed by sucking nutrient carrying juices from infested plants. Soybean aphids directly compete with the plants for the nutrients they need to grow and produce seed. The largest yield losses occur when aphids combine with some other plant stress. For example, losses will be much higher if aphid stress is compounded by drought stress.

The worst outbreaks seem to be occurring every second year: 2001, 2003 and 2005 all had major outbreaks. Ontario Ministry of Agriculture, Food and Rural Affairs advisors, however, are not yet convinced whether the alternate year outbreaks are cyclical or perhaps due to a combination of factors. There were isolated problems in 2007 but they were not widespread. No one is quite sure why, but one theory is that severe cold temperatures in April likely reduced the overwintering population.

Alternating infestation years is likely an indication of the predator cycles. Ladybugs and perhaps more importantly, their larvae, are vicious aphid predators. They alone will often be able to control a moderate outbreak. Since 2007 surprisingly did not have a large outbreak, no one is quite sure what 2008 will bring.

“In winter meetings, I joke with producers that they will definitely need to purchase a new pair of running shoes before spring,” McDonald says. “I don’t think they always appreciate my attempt at humour but that’s the reality. They will have to start scouting fields thoroughly in mid June and depending on conditions, continue scouting regularly until the end of July. Aphids are born pregnant so numbers can explode quickly. If you aren’t keeping an eye out for them, they can become a problem in a hurry.”

If aphids do become a problem, they can be effectively controlled by timely spray applications in mid July to early August. Since they are primarily found on the underside
of leaves they can be difficult targets. Spray applications can be done with either an air or a ground sprayer. Higher pressures and water volumes should be used to ensure maximum coverage. -end-