Two-Spotted Spider Mites and Soybeans: Damage, Scouting and Intervention Thresholds

August 2, 2021

The two-spotted spider mite is a species of phytophagous mites that feeds on vegetation. Of all spider mites, this species causes the most damage to soybean crops, especially in hot, dry summers.

In this article, you will learn more about the damaged caused by this pest. We will also explain how to scout for its presence and the different intervention thresholds that apply.

Damage caused by the two-spotted spider mite

The two-spotted spider mite colonizes the underside of soybean leaves, and in rare cases during heavy infestations, the upper side. They are known for spinning webs on the leaves and stems they infest. An infestation typically starts at the bottom of the plant and moves toward the top as it progresses.

This mite uses its sharp mouthparts to pierce the leaves’ epidermis and extract their cell content. This manner of feeding reduces the soybean’s photosynthetic capacity, which results in lower chlorophyll content in the plant’s cells and a potential loss in yield.

The two-spotted spider mite’s bite creates small pale stains that appear as white or yellow spots, leaving visible stippling under the leaves’ surface.

The lesions made by this insect lead to stomatal closure, reducing the soybean’s photosynthetic capacity and weakening the plant’s leaves, which will wither and potentially drop prematurely. In severe infestations, you can see changing colouration in the foliage, which turns bronze in affected areas.

When the two-spotted spider mite attacks open flowers, it causes a browning and withering of the petals that resembles a pesticide burn. In the worst cases, you will observe quick wilting of the plants along with complete defoliation.

Losses in yield caused by the two-spotted spider mite at the end of the vegetative stage and beginning of reproduction can affect 40% to 60% of the soybean production which, once damaged, may ripen prematurely. The pods of infected plants are more susceptible to shattering or producing smaller seeds, further contributing to crop loss.

Scouting to better identify the issue and extent of the infestation

Two-spotted spider mite infestations usually start at the edges of fields and in raised areas, which are more susceptible to dryness. To detect the presence of this insect in soybean crops, shake the plants above a white surface like a piece of paper and look for mites that have fallen on the sheet. 

Another scouting method is to examine the underside of the leaves from the base of the plant to the apex using a high-power magnifying glass (from 10x to 15x) to look for the presence of mites or webs.

The R1 to R5 growth stages are the best times to scout, and scouting is strongly recommended in hot, dry weather conditions without heavy precipitation in the forecast, which is favourable to the insect’s growth. Note that fields that have had prior infestations are at a higher risk and must therefore be monitored very closely, especially at the start of the season.

The intervention thresholds for protecting soybean populations

Québec has not established intervention thresholds for the use of insecticide or miticide treatments. However, in Ontario, treatment is suggested as soon as soybean populations have an average of four mites per leaflet or when a single leaf per plant is damaged and dryness persists.

To determine whether an insecticide or miticide treatment is appropriate, you can refer to the 6 stages of the scale developed by Potter and Ostlie. This scale is used to assess the degree of infestation and determine the right time to apply treatment, which will help protect the top two levels of the crop canopy. 

  1. No mites or damage observed.
  2. No premature yellowing and minor stippling on under the leaves.
  3. Stippling under the leaves with small areas of yellowing.
  4. Spray threshold—Heavy stippling under the leaves progressing into the middle canopy. Scattered mite colonies present in the upper canopy and mites in the middle canopy. Under leaf yellowing and some leaf loss.
  5. Economic loss—Under leaf yellowing and significant leaf drop. Stippling, webbing and mites in the middle canopy. Upper canopy colonized by mites with minor stippling.
  6. Leaf loss close to the plant with leaf yellowing or browning moving up the plant into the middle canopy. Stippling and distortion of upper leaves. Mites present in high levels in the upper and middle canopy.

Note: scouting must be carried out every four or five days as mite populations can increase rapidly. 

Two-spotted spider mites: small but destructive

For more details on how to intervene and what insecticide or miticide treatment is best for you, feel free to contact us—we’re happy to help!

Have a great harvest!