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Weather, pests & diseases

Midwest farmers woke up to some unusually unwelcome guests last summer in the form of spider mites. More commonly found in Western soybean and grain sorghum fields, spider mite populations suddenly exploded last summer, and the insects feasted on soybean and cornfields stressed by the drought. They may be tiny (you need a magnifying glass to see them), but fields hit hard suffered large yield losses ranging from 40% to 60% in soybeans and 20% to 30% in corn.

Last summer offered ideal conditions for spider mite eggs, nymphs, and adult mites that seemingly emerged all at the same time in a massive population explosion, says University of Wisconsin entomologist Eileen Cullen.

Weather wake-up call

Unusual weather patterns – whether they be drought or deluge – are a tip-off to potential bug or disease problems. For example, an unusually mild winter (like the one a year ago), severe drought (like last summer’s), or prolonged periods of rain and cool temperatures in the spring are an advance warning to anticipate certain pest or disease problems.

The relationship between disease and weather cycles is the base of many prediction models that can be used to advise growers days or weeks before the onset of a disease, says Kansas State University plant pathologist Karen Garrett. She says research has been successful at tracking the emergence of bugs based on weather patterns.

Armed with such weather information this spring and summer, you should head to your fields and “scout, scout, scout,” urges Ron Hammond, Ohio State University.

Cold-blooded at heart

As insects are cold-blooded, they respond directly to temperature changes. Certain insects favor hotter, drier weather, says Sue Blodgett, Iowa State University. So an unusual temperature pattern (warm winter followed by a hot spring, for example) can be used with considerable accuracy to predict the emergence of certain insects.

A good example of this happened last year. A mild winter was followed by a warm spring followed by a hot, dry summer. “The mild winter improved the survival of some insect species, such as corn flea beetles, bean leaf beetles, soybean aphids, and white grubs that overwinter in Illinois,” says Mike Gray, University of Illinois.

Last year, a number of bugs – such as black cutworm, true armyworm, and potato leafhopper – migrated from Southern areas to the Midwest due to winter weather that encouraged these bugs to take a Northern vacation. When they arrived, they were able to feast on crops stressed from a drought, which made the plants less able to fend off feeding.

There was another telling feature of the spider mite invasion. The fungus that attacks them (and keeps the mite population in control) was suppressed due to dry weather.

Diseases more certain

The relationship between weather patterns and the emergence of certain diseases is even more certain. So much so that disease outbreaks can be predicted one to three weeks or more in advance, report plant pathologists at the University of Illinois.

A partial list of diseases that can emerge during a hot, dry summer (particularly following a warm winter) include common smut (in corn) and charcoal root rot (in corn and soybeans).

Diseases likely to occur as a result of a wet, warm spring and summer include foliage diseases, stalk rots, eat rots (in corn), phytophthora root and stem rot, and pod and stem blight (in soybeans).

There is no one piece of advice that covers all weather situations when it comes to predicting the onset of bugs or diseases in your area. Your best bet is to seek advice and warnings from your state agricultural agronomists and area crop consultants.

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