Soybeans: Two-Spotted Spider Mite

Excerpt from Agronomy Guide for Field Crops (Chapter 4)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Description
2. Life History
3. Damage
4. Scouting Technique
5. Action Threshold
6. Management Strategies
7. Updates on Soybeans: Two-Spotted Spider Mite
8. Related links...

Description

Plate 59. Two-spotted spider mites are barely visible to the naked eye. Adults are eight-legged, yellowish-brown, with two dark spots.

Life History

Spider mites generally overwinter as adult females in sheltered areas such as plant debris and field margins. Wheat fields underseeded to red-clover are another important overwintering site. Red clover provides food for mites until freeze-up, allowing the mites to survive right in the field. In late April as the weather turns warm, mites become active in search of food and egg-laying sites. Spider mites disperse by crawling, so infestations tend to spread slowly from field edges. Non-mated female mites will mass at the top of the plants and spin webs that serve as a "balloon" allowing strong winds to pick them up and carry them off to another site. Spider mite females can reproduce without mating. A single unmated female can be the start of a new colony. Under hot, dry, windy conditions, infestations can spread very quickly. There can be up to seven generations per year, with generation development overlapping. Frequent rain and cool weather reduce mite populations in soybeans.

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Damage

Mites feed on individual plant cell contents on the underside of leaves through stylet-like mouthparts. Each feeding site causes a stipple. Severe stippling causes yellowing, curling and bronzing of the leaves. Upon close examination, fine webbing on lower surfaces of the foliage can be seen. Symptoms can be confused with water damage or herbicide injury along field borders. Damage is more severe in hot, dry weather and usually occurs in mid-July after winter wheat harvest. High-risk factors include neighbouring winter wheat stubble fields, hay fields and ditch banks and fencerows that harbour overwintering mites. No-till soybeans following winter wheat underseeded to red clover are also at risk.

Plate 60. Spider mite injury is most visible on the underside of the leaf causing yellowing, curling and bronzing of leaves.

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Scouting Technique

Scout fields around the first week of July. Infestations usually move in from the edge of fields as hot spots. Look for tiny white stipples on the upper surface of leaves in the mid-canopy. Turn these leaves over and look for slow-moving dark specks (a 10X-hand lens may be necessary to see mites).

Action Threshold

Four or more mites per leaf or one severely damaged leaf per plant prior to pod fill is cause for concern.

Management Strategies

If mite numbers exceed the action threshold, an insecticide may be necessary. Refer to OMAFRA Publication 812, Field Crop Protection Guide, for recommended products. Early infestations may be kept under control by using border sprays. This will help prevent the spread of mites to other parts of the field and may reduce the need for further treatment. If rain is in the forecast, spraying should be delayed; prolonged wetness will usually reduce the number of mites to insignificant levels. Use of drought-tolerant varieties will minimize the effect of spider mites. Natural enemies help keep mites at low levels when conditions are unfavourable for the mites. Natural enemies of mites include ladybird beetles, thrips and predaceous mites. Cool temperatures and high humidity can promote a pathogen to develop that can also provide natural control. (Order OMAFRA Publication 812)

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Updates on Soybeans: Two-Spotted Spider Mite

• Field Crop Protection Guide - Publication 812
  • Soybeans: Two-spotted Spidermites

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Related links...

• Insects
• Soybeans

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