New Utah Pests Fact Sheets:

• Cankerworms
• Cucumber Beetles
• Curly Top of Tomato
• Necrotic Ring Spot
• Onion Powdery Mildew
• Onion Thrips
• Subterranean Termites
• Snow Mold on Grains
• Using Degree Days

UTAH PESTS MEMBER
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Unusual Spring Weather Presents Unusual Utah Pest: Winter Grain Mite 
 

In April, a “first time” sample of winter grain mite (Penthaleus major) was submitted to the Utah Plant Pest Diagnostic Lab (UPPDL) by a soft white wheat grower in Box Elder County.  Winter grain mite is distributed throughout North America, but requires more moisture than warm season mites and is not a frequently pest in the arid states of the West.  This year’s cool moist spring conditions, however, were prime for its population growth.  Winter grain mite causes damage to small grains, including wheat, barley, oats, rye, ryegrass, bluegrass, bentgrass, fescue, vegetables, legumes, ornamental flowers, cotton, peanuts, and various weeds.

Up close, damage from this mite looks like white or gray specks on the leaves.  This is caused by their piercing mouthparts, which puncture individual cells, allowing the contents to be consumed.  From afar, the small grain or grass field will have a yellow/gray/silver cast, showing up in winter or early spring.  Overall damage to plants can be caused from feeding on the base of the plant by larvae and adults.  Heavy pressure from these mites can cause stunting and reduced yield.

	Life cycle of winter grain mite.

Winter grain mites are not insects, but rather small arachnids related to spiders and ticks.  They have eight legs, no wings, and piercing mouthparts. Compared to other mites they are relatively big, about 3/64-inch long.  These mites can easily be seen with a 10x hand lens.  They are black with red legs, and a red section toward the rear of their body on the back (shown above).  They are the only cool season mite that has this appearance.

Winter grain mite has two generations per year.  Depending on temperature and moisture, the first generation hatches from over-summer eggs in late September through early November.  The mites take about 35 days to reach maturity.  Winter grain mite females are asexual, and can lay around 30 eggs during her short (~35 day) adult life.  The winter eggs take about 25 to 35 days to develop before they hatch (2nd generation).  These mites reach maturity and lay the over-summer eggs.  Winter grain mite populations (and damage) peak in December/January, and again in March/April.

Knowing the habits of winter grain mite can help you control this pest.  First, the eggs are laid by the female on the soil or on the base of leaf sheathes.  The larval stage feeds on the base of the plant.  The nymphs and the adults can feed around the base of the plant, but on cool, cloudy days and at night, they will feed higher up on the plant.  Generally, it is better to search for mites at night.  When approached, however, they will fall from the plant to the soil where they are difficult to see.  Optimum temperatures for egg hatch are between 45°F and 55°F, and between 40°F and 75°F for adult feeding.  When temperatures fall below, or exceed these ranges, mites retreat to the soil.  In the case of severe temperatures or moisture deficits the mites can move 4 to 5 inches into the soil.  Winter grain mite populations peak in winter and spring and they spend the summer in the egg stage.  In the short term these mites are capable of withstanding sub-freezing temperatures, ice, and snow.  During periods of persistent snow cover, mites can feed continuously on plants under the snow.

Controlling any mite species is never an easy task.  Because they are so small and have such a high reproduction rate, they are able to develop pesticide resistance.  Spraying chemicals for mites should always be a last resort.  Because winter grain mites do not travel far and over-summer in the egg stage, cropping practices such as tilling and crop rotation can be very effective at keeping mites below economic threshold levels.  Crops such as cotton, corn, clover, or sorghum are good choices for rotating. Never grow small grains for more than 2 consecutive years in the same field (if mites are a problem).  To stop the spread of mites from field to field, thoroughly wash equipment to remove mites and eggs.

Use caution when treating with pesticides.  Most materials registered for winter grain mite are broad-spectrum, which will provide initial control, but at the same time, kills beneficial predatory mites such as Balaustium spp. and other bio-control arthropods. As a result, using these types of sprays can result in a subsequent flare-up of mite populations.  If a pesticide is necessary, it is best to spray when the mites are on the tops of the plants on cool, cloudy days.  Spraying on days that are too hot or cold will yield poor control because the mites are safe below the soil surface.  Some pesticides registered for use on winter grain mites include dimethoate, methyl parathion, chlorpyrifos (Dursban, Lorsban), and malathion.  Using pesticides in combination with rotation could greatly reduce mite populations over time, making only rotation necessary in the long term.

-Ryan Davis, Arthropod Diagnostician

Picture of the Quarter 

In last quarter’s newsletter we showed a picture of herbicide injury in a greenhouse setting.  Here is another case, only in a landscape setting.  This pine tree was damaged by a nearby application of the broadleaf herbicide 2,4-d.  It could have been caused by drift or soil uptake.  Herbicide damage includes stunted growth, yellowed leaves, and cupped or distorted foliage.  This pine tree will recover from the injury. -Photo by Marion Murray