Keep a close eye on pests
By Darren Clark CCA-ON, 4R NMS
Crop Sales Specialist
AGRIS and Wanstead Farmers Co-operative
Now that the crop is nicely progressing with the warm temperatures and variable rainfall across the trading area, it is now the time to get out and scout. Depending on where you fall within our trading area, this hot and dry weather doesn’t just bring on crop stress - it also increases the risk of an infestation of spider mites and aphids in soybeans.
Two-spotted spider mites
Spider mites are very small and are barely visible to the naked eye. Adult mites are roughly 0.5 – 1.0 mm in length, rounded, eight-legged yellowish brown with two dark spots on the sides of the abdomen.
(Photo) Two-spotted spider mites.
(Photo) Two-spotted spider mites and eggs.Spider mites are sucking insects that feed on the individual plant cell contents on the underside of the leaves. The feeding takes away the plant nutrients and water which can cause significant damage, especially in drought-like conditions. Each feeding site causes a stipple-like appearance, giving the upper side of the leaf looking like it was sandblasted. Severe stippling, curling and bronzing of the soybean cause the leaves to dry up and eventually fall off.
(Photo) Stippling symptoms on upper surface of a leaf.High-risk conditions for spider mites include a dry weather forecast, adjacent harvested wheat fields, and recently mowed ditch banks. An important one to note is wheat fields under-seeded with red clover as the red clover provides food for them. Infestations tend to occur shortly after wheat harvest and when municipalities mow roadsides.
(Photo) Two-spotted spider mite infestation moving in from field edge.Scout weekly
Starting the first week of July and especially after wheat harvest, scout weekly. Infestations typically move in from the edge of fields as hot spots. You will need a 10X hand lens to actually see the mites. Look for tiny white stipples on the top of the leaves in the mid-canopy. Take these leaves from the plant and shake them onto a white piece of paper to see the mites moving around. Also, inspect the leaves for eggs.
The threshold for spider mites is four spider mites per leaflet. If mite numbers exceed the threshold, an insecticide may be necessary. If there is rain in the forecast, delay spraying as heavy rain will wash them off.
For an insecticide application, stick to dimethoate, as a pyrethroid insecticide (Matador) will kill beneficial insects and cause spider mite populations to increase. In most cases, an insecticide application around the perimeter of the field is all that is needed.
Soybean aphids
Soybean aphids are small pinhead-sized pale yellow aphid with black cornicles and a pale-yellow tail. Adults may be winged or wingless.
(Photo) Wingless adult soybean aphid. Source: A. Schaaftsma, University of Guelph, Ridgetown Campus.
(Photo) Soybean aphids are yellow-bodied insects with distinct black cornicles, and may be either winged or wingless. Photo by J. Obermeyer.Aphids require two hosts to complete its life cycle. They first lay eggs on Buckthorn twigs in the fall to overwinter. Nymphs hatch from these eggs and the aphids go through two generations as wingless females on Buckthorn. The third generation develops as winged adults that migrate to early planted soybean plants. They continue to produce wingless generations until the soybean plants become crowded with aphids. After that, they produce another winged generation that takes off for another suitable plant to colonize. There can be up to 18 generations in a single year. Southwestern Ontario has few Buckthorn trees meaning we typically don’t see aphid infestations until later in the season (R3 or beyond).
(Photo) Aphids on the underside of the soybean leaves.Aphids are piercing-sucking insects that suck the nutrients from the soybean leaves and stems. They are usually attracted to plants under stress, as they have high amino proteins trapped in the leaves, making them an excellent food source for the aphids. They will produce a sticky waste and under high pressures, this waste can cover the entire plant.
Yield losses are greater when infestations occur in R1 and R2 growth stages as early stresses can reduce flowering and pod formation.
Infestations at R3 to R5 can cause yield losses as bean seed size and weight can be reduced.
To scout, early-season aphids tend to concentrate on the newly emerging leaves and upper trifoliates of the plants. As the season progresses, the aphids tend to migrate down to the middle of the canopy. Check fields every 7-10 days from early July until the end of August scouting 20-30 random plants in the field.
The action threshold is 250 aphids per plant and increasing numbers. This gives us 7-10 day lead time before aphids would reach an economic injury level where the cost of control will equal yield loss. The actual economic threshold is 600 aphids per plant. Under dry conditions, we may have to take action sooner due to added stress.
Before making an insecticide application, ensure there are no spider mites present.
As always, if you have any questions or concerns with anything you are seeing in your fields, contact your local AGRIS or Wanstead Co-operative.
Reference Source OMAFRA Pub 812.