BRANDON – Pea aphids might be puny compared to voracious grasshoppers, but the fast-breeding bugs can significantly lower pea yields.
At only an eighth of an inch long, the green, soft-sided insects can be hard to spot and it takes serious scouting to determine whether they are numerous enough to warrant spraying, said Bruce Brolley, Manitoba Agriculture pulse crop specialist.
“This is a pest we go after and yes, it is a pest that we can get some economic returns on, but it has to be sprayed at the right time,” he told farmers attending Manitoba Ag Days held Jan. 20-22 in Brandon.
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However, the economic threshold for spraying changes depending on the crop’s yield and pea prices. At today’s low pea prices, it might be wise to let the aphid’s natural predators limit the crop damage.
The aphids have some unusual reproductive attributes, he said.
The females do not lay eggs. Rather, they give birth to live offspring. Each aphid produces 50 to 150 offspring in its life. These bugs mature in five to 50 days, depending on weather. The warmer it is, the faster they mature.
In a year, they will produce seven to 15 generations.
“They are quite amazing in their reproductive ability. That is one of the reasons why you have to scout for pea aphids on a weekly basis. If you get some warm weather, these things can take off on you very quickly,” Brolley said.
The aphids spend the winter on alfalfa and clover crowns. The offspring normally can’t fly, but when there is an interruption in plant sap that they live on, they give birth to winged females that fly to greener pastures.
That’s one way they get into pea fields, but they can also come from the United States, blown on the wind.
They do little damage when the plant is in its vegetative state, but can be a big problem when it is setting its pods.
“Unlike caterpillars that leave obvious symptoms in the field, these things that are sucking on pea leaves, typically, there are no symptoms. The plants don’t raise a flag and say, ‘help me, aphids are here and are attacking me.'”
The key is determining their number when the field is at about 50 percent bloom. At that stage it is about seven to 10 days before the point where the plant is most vulnerable and should be sprayed.
If the aphids are a problem at the 50 percent bloom stage, the farmer has about a week to acquire insecticide and sprayer.
When scouting, Brolley recommends walking to the centre of the field and then walking in a “W-shaped” route, sampling at five locations.
While aphid numbers can be determined with a sweep net – nine to 12 aphids per 180 degree sweep is the threshold – it is hard to count the bugs in the net, Brolley said.
He prefers slapping the top eight inches against a piece of paper. Test about 20 plants per location.
If there is an average of one to two per plant through the field, it could indicate the potential for almost five percent damage and signal the need to spray, if the potential revenue in the field warrants it, he said.
If there are only one or two hot spots, it probably isn’t worth spraying.
