Spider Mites in Field Crops

Agdex#:	141/620
Publication Date:	01/89
Order#:	89-002
Last Reviewed:	01/89
History:
Written by:	H. Olechowski - OMAFRA; A. Schaafsma - Ridgetown College of Agricultural Technology

Table of Contents

1. Biology
2. How They Spread
3. Importance of Weather
4. Soybeans
5. Field Corn
6. Control

The two-spotted spider mite is an occasional pest of soybeans and corn in southwestern Ontario. Local
outbreaks of spider mites occur annually; however, since the mid-1980 s damaging outbreaks have been common in Essex and Kent counties.

Damage is particularly severe in dry years as was the case in 1988 when spider mite infestations spread over thousands of acres in southwestern Ontario and caused substantial yield losses. The first field symptoms usually become visible in soybeans in mid-July, after winter wheat harvest, appearing similar to water damage or herbicide injury along field borders. If the mites go unchecked, damage could be severe.

Figure 1. A magnified view of a two-spotted spider mite adult. Notice the clear, round egg beside the adult.

Biology

The two-spotted spider mite is more closely related to spiders than to insects. It has five developmental stages: (a) a clear round egg; (b) a larval stage with three pairs of legs; (c) two nymphal stages. (d) each with four pairs of legs; and (e) an adult stage with four pairs of legs. Adults are extremely small and barely visible to the naked eye (Figure 11. Two-spotted spider mites produce webbing but unlike the webbing of a spider, the underside of an infested leaf has a sandblasted appearance.

The mites overwinter as adult females in protected areas such as plant debris in hedge rows, fence rows and fields with heavy trash. During this stage, the two-spotted spider mite, which is usually clear with two spots, turns reddish orange in response to cooler fall and winter temperatures (Figure 2).


Figure 2. Overwintering reddish orange stage

The onset of warm spring weather activates the mites in search of food plants and egg laying sites. Each mated female will lay approximately 100 eggs over a 30-day span. It is not necessary for these mites to mate; therefore, it takes only one female to establish a new colony at a new location. Under ideal conditions mated and non-mated spider mites can complete their life cycle in less than a week; therefore, their population can explode in a short period of time. There can be 10 to 15 generations per season.

Two-spotted spider mites will feed on an extremely wide variety of host plants, including many grassy and broadleaf weeds, fruit crops, vegetable crops, and field crops such as soybeans and field corn. They usually feed on the underside of the leaves by piercing and sucking plant juices.

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How They Spread

Tivo-spotted spider mites do not have wings, and are therefore limited in their ability to disperse. Mites travel short distances by crawling, but depend on wind for long-distance dispersal.

When the food supply is adequate mites tend to stay in the lower protected parts of the crop canopy. Crawling seems to be the more important mode of dispersal early in the season when the mites move in from bordering areas into the crop. Movement by wind becomes more important later in the season, which often coincides with corn tasseling time. When the food source of the host becomes less suitable, the non-mated female mites will begin to mass at the top of the plants. The mites are able to spin webbing, and often this webbing will serve as a balloon , allowing even small winds to pick them up and carry them for quite a distance. This explains the appearance of mites throughout a crop where none were observed a short time before.

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Importance of Weather

Hot humid conditions with little precipitation favor spite mite outbreaks. These are often coupled with high winds which disperse the mites great distances. Although a heavy rainfall will wash many mites off leaves and drown them, a fungus disease that develops in the humid conditions following a rainfall is more important in the decline of mite populations.

Figure 3. Typical spider mite feeding damage on soybeans spreading in a semi-circular pattern from the point of infestation.


Figure 4. Spider mile feeding damage results in a ' stippled ' appearance of the upper leaf surface.

Figure 5. Feeding damage on the underside of a leaf has a sandblasted" appearance.

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Soybeans

Spider mite damage in soybeans usually begins in mid-July along field margins, neighbouring winter wheat stubble fields, hay fields, ditch banks and fence rows, and may continue to the end of the growing season. Damaged areas will spread in a semi-circular pattern into the field from the original point of infestation (Figure 3). 'Hot" spots may also occur in the middle of the field if the mites are blown in by strong winds.

Damaged leaves will begin to yellow, then bronze and eventually fall off the plants. Under light spider mite infestations, the upper leaf surface has a stippled" appearance (Figure 4) and the underside leaf surface has a sand blasted appearance (Figure 5). A damaging threshold has been reached if pod fill is not complete and there are 4 or more mites per leaf or one severely damaged leaf per plant. This threshold may occur 40 to 60 meters from a hot" spot.

To check for mites, sharply tap a plant against your hand while holding it over a white sheet of paper. Look for slow moving dark specks. You may need a lens to see the mites clearly.

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Field Corn

Two-spotted spider mites seldom cause economic damage in field corn. Moderate infestations will result in a stippling of the leaves with yellowish-brown spots. Severe infestations will cause entire leaves to turn yellow and brown. Look for evidence of mites on the underside of the leaves, either the mites or the webbing they leave can be seen with the naked eye. A pocket hand lens would make identification much easier.

If the corn is in late dough or early dent stage, it is unlikely that there will be an economic benefit to a
chemical control measure, even during a severe mite infestation. If the corn is in the milk or early dough stage and the mites are severely discolouring the leaves below and near the ear. control measures should be taken. This is extremely rare in southwestern Ontario.

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Control

For up-to-date control strategies refer to your local Ontario Ministry of Agriculture, Food and Rural Affairs Office or OMAFRA Publication 296. Reid Crop Recommendations.

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