Scouting for Aphids

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Adult aphids are approximately 1.5 mm in length and are a pale greenish-yellow insect with black cornicles (“tailpipes”) and a pale yellow “tail”.  Adults may or may not have wings.  Nymphs look exactly like adults but are smaller and do not have wings.

The primary host for aphids is European buckthorn but the primary crop for feeding is soybeans.

Life Cycle

There can be several generations of aphids per year.  They overwinter on European buckthorn and nymphs hatch in the spring and molt into adults.  Two generations live on the buckthorn before the third generation develops wings and can fly to colonize soybeans.  These aphids continue to produce wingless offspring until they become crowded, and then winged adults are produced and can disperse to other plants or fields.  Only females are present throughout spring and summer months, winged males and females are produced as fall approaches.  This allows them to fly and mate to lay eggs.

Damage

Aphids have piercing mouthparts which they use to suck juice and nutrients from plants. Once populations have reached threshold levels, aphids can cause the plants to abort flowers and become stunted, which reduces seed production and quality.  Yield loss from aphids is greatest when soybeans are in their early R-stages (R1 and R2), when flowers will abort, negatively impacting pod establishment.  Infestations in pod-fill stage (R3) and beyond can result in reduced seed size and quality.  Typically, aphid injury is worse in dry years when plants are stressed.

At Risk Areas

Any field can be at risk of aphids each year.  Early planted fields are prone to early season infestations from aphids moving from buckthorn in the spring, though this risk is often lower in areas where buckthorn is scarce.

Late planted fields are prone to summer migration of adults coming from other soybean fields.  In addition, because aphids feed by sucking water and nutrients from plants, fields under drought stress or potassium deficiency can be more prone to injury.

Scouting & Thresholds

When scouting for soybean aphids, look at 20 random plants throughout the field, avoiding field edges.  Turn over leaves on each trifoliate of the plant to count the number of aphids on leaves and stems – estimate the number of aphids per plants and note the crop stage.  Fields should be checked every 7 to 10 days from the beginning of the R1 stage until the crop is well into the R6 stage and checked more frequently (3-4 days) as populations approach threshold.  A minimum of two field visits is required to confirm whether populations are increasing.  It is also important to note the presence of natural enemies (lady beetles, lacewing larvae, minute pirate bugs, syrphid fly larvae).

From the R1 to R5 stage, 250 aphids per plant and actively increasing on 80% of plants checked is considered threshold and action should be taken.  This gives 7 to 10 days before aphids would reach the economic injury level where cost of control is equal to yield loss.  If aphids are at 250 per plant and not actively increasing you can assume that natural enemies are keeping up with the population.

Pest Management Strategies

Removal of buckthorn can eliminate the overwintering host.   Otherwise, if present, natural predators can help to control populations or chemical control options can be used.  Foliar insecticides can be applied once threshold has been reached.  Fields should continue to be scouted after spraying to ensure that aphid populations do not rebound or secondary pests do not flare up.  Be careful to select insecticides that will not harm beneficial insects, such as bees, as they forage during flowering.  Neonicotinoid seed treatments also help with early season control of aphids.

 

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Sources: Christina DiFonzo, Michigan State University, Bugwood.org. Grain Farmers of Ontario. Guide to Early Season Field Crop Pests. (verified May/15).