Weather and Climate

Harlow B. Mills.

Within broad limits, climate governs the general distribution of insects. Weather affects the numbers of insects within their areas of distribution and the fluctuations in numbers around the margins of the areas.

Weather is a snapshot photograph of the atmospheric conditions winds, temperature, air pressure, precipitation, and humidity at any one time. Climate is a composite picture of those conditions over a longer period. When we consider weather in relation to insects we are generally considering the effects of extremes of a variable atmosphere. When we think of climate we are thinking primarily of average conditions.

Because weather and climate are expressions of the same phenomena, differing only in time, it often is difficult arbitrarily to divide their effects on the insect populations.

Insects are limited in space and in numbers by many factors, some of which are not directly associated with weather or climate. A parasitic insect may be limited by the absence of its host animal, or a plant-attacking species may not occur in an area because of the absence or rarity of the plant upon which it depends. But in the main an insect is so completely imbedded in atmospheric factors that they must be considered as of great importance in controlling its occurrence and abundance.

Some of the factors composing weather and climate are obvious. Others are neither obvious nor easy to measure. Possibly others are unmeasured or undiscovered as yet. Often the factors work in intricate combination to affect living things.

But some factors are so conspicuous that they can be isolated to show that one cause will produce one effect. In a given area, for example, prolonged spring rains may produce many breeding pools and hence more mosquitoes than a dry spring with a lack of breeding pools. Heavy, pelting rains in the early summer may be the single factor that might end a threat of an outbreak of chinch bugs by destroying the small nymphs. Prolonged high temperatures may allow for an increase of house flies.

Unusually low summer temperatures may greatly reduce the numbers of an insect. That happened in 1950 when prolonged cool temperatures in northern Illinois reduced the second generation of European corn borers by allowing only about io percent of the first-generation borers to become pupae and to emerge as egg-laying moths, instead of the 50 to 80 percent that usually furnish the borers of the second generation. Thus, because of an abnormally cool summer in the area in 1950, a second generation of borers and consequent second-generation damage were not important.

Weather effects on insects often act over a short period, and the critical period may be missed in the field and completely hidden in the published weather data. In a day, or a few days, weather factors may reduce a population of insects capable of devastating a crop to one of no economic importance. Often investigators have overlooked this factor of timeliness, although in such insects as chinch bugs and grasshoppers it can be most important.

BECAUSE THEY ARE more easily controlled in the laboratory and measured in the field, temperature effects on insects have been studied more intensively than have the other of the obvious climatic factors. As insects are "cold-blooded," they respond directly to temperature changes so directly, indeed, that temperatures can be ascertained with considerable accuracy by certain insect activities. For example, various insects' songs can be translated directly and with little error into temperature readings by the use of certain formulas. The snowy tree cricket emits high-pitched, tremulous chirps, each separated from the next by a pause of approximately the same duration. If you count these chirps for a minute, you can determine the temperature in degrees Fahrenheit by adding 40 to one-fourth of the number of chirps per minute. Thus, if there were 120 chirps per minute interval, the temperature would be 40 plus one-fourth of 120, or 70 F.

Insects may be active over a wide range of temperatures. S. W. Frost in his General Entomology wrote that certain soldier flies can live in hot springs where the waters are at 122 . J. H. Pepper and I reported in the Annals of the Entomological Society of America that the alpine rock crawler came to rest at a temperature of about 38 when given a choice.

Those extreme cases are evidences of insect versatility, but most species are active at intermediate temperatures and are affected by the day-to-day and year-to-year play of more usual and more expected temperatures. Warm and long summers allow the southern house mosquito to move far north, and the reverse situations push it south a direct expression of the effect of temperature on the distribution of a pest.

Most of the overwintering larvae of the brown-tail moth are destroyed by temperatures approximating 25 ; such minimum temperatures rear a wall of cold against northward dispersal. Likewise the average annual minimum isotherm of 15 marks roughly the northern limit of the San Jose scale.

Insects of tropical distribution seldom can withstand freezing temperatures. Those living in cold areas usually can exist under winter conditions only in one stage; the others are susceptible to destruction by low temperatures. Thus only the adults of chinch bugs can live through the winter, only the larvae of the brown-tail moth, only the eggs of the gypsy moth, and only the pupae of the tomato hornworm.

The female of the snowy tree cricket inserting her eggs into a raspberry cane.

The relationship between temperature, rate of growth, and distribution of insects is demonstrated by R. L. Shotwell, of the Bureau of Entomology and Plant Quarantine, who studied the development of several species of grasshoppers from the Great Plains.

The clear-winged grasshopper developed much more rapidly than the others he studied. That may explain why it is an important pest at high elevations and northern latitudes where the growing season is short. The tremendous infestations of this species which have occurred in such high mountain areas as the Centennial Valley of Montana illustrate the point. The lesser migratory grasshopper also has a rapid rate of development and thus a possible adaptation to a short growing season. It inhabits northerly regions and high elevations as well as lower and more southerly areas, where sometimes it has two generations in a season.

An interesting temperature relationship is illustrated by the two-striped grasshopper (Melanoplus bivittatus) and the differential grasshopper (M. digerentialls). The two species are rather closely related in their morphology and habits. Both inhabit the same area, but the two-striped grasshopper extends far north and west of the other in the Great Plains-Rocky Mountain region.

Shotwell found that a sample of eggs of the two-striped grasshopper hatched in an average of 8.6 days at 77 , while a sample of differential eggs hatched in 22.5 days at the same temperature. A similar spread occurred at incubation temperatures both above and below 77 . He discovered when nymphs were reared that the period of nymphal development at 77 averaged 39.2 days for the two-striped grasshopper and 50.6 days for the differential.

On that basis, Shotwell concludes : "Not only the eggs but the nymphs of M. bivittatus make more rapid development at all temperatures than do those of M. differentialis ... an earlier hatch and more rapid nymphal development enables M. bivittatus to develop in outbreak numbers in latitudes much farther north than those at which M. digerentialis can do much damage."

What is the significance of these observations?

In the mid-1930's a series of exceptionally long, hot summers enlarged the area where the differential grasshopper could succeed. Making the best of the situation, the species appeared both north and west of its previous range. Many of the extensions of range did not continue, but an invasion into the lower reaches of the Yellowstone River has continued to exist, often in injurious numbers. The populations largely are limited to the more protected river bottoms and make less headway on the exposed uplands.. That may not be entirely an effect of temperature, but anyway the two species have been shown to be limited in distribution in certain directions by the prevailing temperatures.

Shotwell's observations show what temperature may do in controlling the distribution of insects. Within the normal area of occurrence, the temperature often affects the numbers of a species by increasing the number of generations in a season and by increasing the number of individuals within any one generation.

This is well illustrated by Dwight Isley, of the Arkansas Agricultural Experiment Station, in his studies on the boll weevil. He learned that when temperatures were increased from 69.8 to 87.8 F., the time required to develop from egg to adult was cut in half. Further, a rise in temperature from 77 to 84 may result in an increase of about 70 percent in the number of eggs laid, and a decrease from 77 to 71.6 may, conversely, result in a reduction of about 50 percent.

RAINFALL in spring, summer, and autumn directly affects the abundance of the hessian fly. James W. McCulloch, of the Kansas Agricultural Experiment Station, found that to be true in four outbreaks of the pest in Kansas. "Three cases ... were years of excessive rainfall," he wrote. "In the case of the outbreak of 1903 there was a superabundance of rain during the spring months and during the time that most of the injury occurred. The decline of each outbreak was accompanied by a decrease in the precipitation, which was generally much below the normal."