Scout Weekly as Soybean Aphids Surge

By Scout Nelson

The heavy rains and high temperatures have held soybean aphid numbers in check through much of the season. However, numbers are now increasing rapidly in some fields, making timely scouting and smart treatment decisions essential. The following guidance summarizes key, research-based practices for in-season management.

Be thorough when scouting. Aphids can occupy the whole plant, not just the upper leaves. Examine entire plants from multiple areas of each field. Scout weekly; when weather favours aphid growth, consider checking more often. Under typical conditions, soybean aphid populations double every six to seven days, but in ideal weather (upper-70s to mid-80s °F) and with few natural enemies, populations can double in just a few days. Fields that received early prophylactic sprays may have fewer predators and parasitoids, allowing faster aphid buildup.

Use the economic threshold of 250 aphids per plant when 80% or more of plants are infested and populations are increasing. This threshold is the trigger to line up an insecticide application within about seven days—before densities reach damaging levels. If counts are below threshold, keep scouting and allow natural enemies (lady beetles, parasitoid wasps, and fungal pathogens) to suppress aphids. “Remember that retail profits, not science, may be driving some of the lower threshold recommendations that you could be hearing.”

Think critically about insecticide selection. Avoid products containing only a single pyrethroid—or mixtures that are essentially multiple pyrethroids—because of widespread pyrethroid resistance. Some chlorpyrifos products are again available and can be effective, but organophosphates have a less favourable human safety profile and, as broad-spectrum materials, will also remove beneficials.

Newer, more aphid-selective options such as sulfoxaflor (Transform), afidopyropen (Sefina), and flupyradifurone (Sivanto) control aphids effectively while being less harmful to natural enemies and often more pollinator-friendly. In many fields, adding a broad-spectrum partner to these selective products does not improve aphid control and can harm beneficials.

For resistance management, mixtures provide limited benefit for clonal pests like aphids. Instead, alternate modes of action between applications. After spraying, re-scout within several days to confirm efficacy; do not assume every application works.

Consistent scouting, science-based thresholds, selective chemistries, and post-treatment checks can protect yield while preserving beneficial insects and slowing resistance.

Photo Credit:istock-oticki