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Learning more about soybean aphids

... introduced to soybean aphids in a rather abrupt fashion.

March 10, 2008  By Ralph Pearce

In 2001, growers in Ontario were introduced to soybean aphids in a rather abrupt fashion. It was a bumper crop, so to speak, and the damage was relatively widespread and severe. The learning process had begun.

Soybean aphids can occur every year, not every other year, so growers must scout more frequently to help with their spraying decisions.

Six years and several infestations later, growers, extension personnel and researchers are still learning. And some of the lessons are advising some adjustments to conventional thinking that developed in the early years following 2001. For instance, one of the widely held notions of soybean aphids being an ‘every other year’ occurrence, is simply untrue for Ontario. That may have been the case within the first three years, but warm winters, sufficient hosts for overwintering and early spring storms that bring aphids into Ontario have combined to make their ‘visits’ an annual event, even if only in patches.

At the 2007 Southwest Diagnostic Days at Ridgetown College, Tracey Baute, field crops entomologist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), spoke of the changing standards. In addition to soybean aphids being an every year pest, she shared some of what she has learned during the past three or four years, particularly pertaining to thresholds for spraying. Since 2001, the threshold standard has been 250 aphids per plant. At that point, it was said, a grower should be out spraying. But now there is a better understanding of what that 250 level signifies.


Leanne Freitag, an agronomist with Cargill, agrees with Baute’s observations. “There is definitely much interpretation to be had when looking at aphid thresholds, including the time of year when those populations are heavy,” she says. “In 2007, they came in earlier than we’ve ever seen them before. So the state of the crop and the health of the crop are both very important to consider when you decide to pull that trigger for spraying.”

The general guideline Freitag follows is the 250 aphids with the population increasing, however, 250 aphids on soybeans that are six inches in height is more stressful to the plant than 250 aphids on soybeans that are knee high. In other words, timing is everything. In 2005, Freitag remembers aphids entering Ontario later in the summer and during a growing season where soybeans were not drought stressed. “There were fields that reached 1000 to 1500 and we sprayed them, and there was a bit of a yield increase from spraying,” she says. “But in 2007, we saw upwards of 20 or 30 bushels per acre increase by spraying, so having that additional stress on the plant allows the aphids to have more of an impact.”

The more scouting, the more understanding
Laura Neubrand is another agronomist who uses the 250 threshold as more of a guideline, however, she holds to it as a default position. For most growers who do not scout, the conventional threshold is a quick and easy figure on which to base their spraying decisions. “If you have someone who is checking their fields constantly, then it depends,” explains Neubrand, who works with Hoegy’s Farm Supply in Brodhagen, Ontario.

“We had one of the first fields in this area that was infested with aphids in 2007 and we held off and held off, and we watched it every single day, and all of a sudden, there was a day you could tell the aphid numbers were increasing and the beneficials were not handling it. That’s the day we went in there and nailed them. Was the threshold over 250? Yes it was, but as a general rule, we do need to stick with that 250 because we don’t check every field as we need to.”

The key to the threshold is preparation. At 250, says Neubrand, growers should ‘get ready’ and monitor the aphid populations during the course of several days, not just scout once and decide to pull the trigger. The challenge here is an old one: growers need to do more scouting of their fields, preferably every five to 10 days, particularly at mid season.

Like Freitag, Neubrand cautions against treating soybean aphids as an odd year pest; there are local hot spots which can happen in any one year. “In our area, some growers have been saying they are a problem once every three years, so aphids aren’t going to be a problem in 2008,” says Neubrand. “But, that’s not the case. It also depends on the weather conditions that we’re dealing with. We’re definitely having more of an issue with them in the drier years. When we get rainfall, that changes a little bit as to how we manage them, but I think we’ve seen them probably two out of the last three years and it was not every other year we saw them; we saw them in 2006 and we saw them in 2007.”

Monitoring the progress of soybean aphids should be done frequently during the growing season, as much to check the control by beneficials as the movements of aphids.

Role of beneficials on the rise
The 2007 growing season also saw the arrival of aphids much earlier in parts of Ontario, particularly along a corridor north of London, from Komoka to Thorndale. Early arrivals coupled with drought stress through the summer months forced many growers to spray earlier than in past years, which is a concern for Baute. As aphids have become a more regular pest in Ontario, populations of beneficial insects, those that feed on aphids, have grown, as well. Ladybird beetles, syrphid fly larvae, minute pirate bugs and parasitoid wasps are usually out in force to meet aphids as they arrive in fields in June or July.

Earlier spraying by growers poses a threat to natural enemy populations for the entire season. “It would be disappointing to see growers start misusing pesticides, mainly because of convenience, time restrictions or their misunderstanding of when to control their aphid problems,” says Baute. “Because that will impact their biological controls and when it comes to parasitoids, and ladybugs in particular, they are very sensitive to pesticide use.”

As part of the learning curve pertaining to beneficial insects, Baute points out that there are three classes: predators, which simply eat aphids; parasitoids, which lay eggs on or inside the aphids to continue their life cycle; and the pathogens which consist of fungi, bacteria or viruses which can infect and kill off the aphids. As more information about the predators and parasitoids is brought forward, there is a great opportunity for growers to begin understanding that there is a degree of biological control, working for them, something Baute says can and must be protected.

In fact, some chemical agents that can kill aphids are begin deregulated in the US and simply are not available in Canada because they can be harmful to the environment, including beneficials. “But we also have a diverse cropping system here in Ontario, where other types of aphids may live and support and allow the parasitoid population to build before they move to soybeans,” says Baute. “All around, we have a good system in place.” -end-

The Bottom Line

This is a very tough decision to spray or not. Aphids have appeared here the last three or four years, but until 2007 we did not spray. Approximately half of our soybeans were sprayed with no noticeable difference in yield. Grahame Hardy, Inkerman, Ontario.

Humbling! Just when you think you’ve got it figured out, the next season is different. One common factor from year to year seems to be that if the soybean plant has an additional stress such as drought or low potassium fertility, then aphids will have an even greater impact on yield, shifting the bias toward control. Alan McCallum, Iona Station, Ontario.


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