Tinkering With Insects' Bioclocks

Dora K. Hayes, research leader, Livestock Insects Laboratory, Beltsville Agricultural Research Center, Agricultural Research Service.

The field of chronobiology deals with quantitating rhythms with periods varying from minutes to years, and sophisticated analyses have been developed to detect meaningful (statistically significant) rhythms in "noisy" data. For purposes of this publication, we accept the fact that rhythms exist and deal with their manifestations.

Travelers who cross time zones usually experience jet lag. Jet lag is a dramatic illustration of the effects of desynchronization of rhythms in these travelers. Insects are small, complex animals. Their biological systems are similar to those of humans, although these systems are considerably smaller and may appear in somewhat different form anatomically. Some of the rhythms in many insect species, including important agricultural pests, include those of oxygen utilization, susceptibility to insecticides, activity, mating, and egg laying.

Insect Rhythms

In humans, regularly recurring cues provided by daylight and darkness, schedules of social activities, meal-timing, and so on serve as synchronizing mechanisms. Humans don't generally realize that rhythms exist in insects or that understanding insect rhythms will help in learning how to manage pest insects. Insects must hatch, pass through definite developmental stages, emerge from the pupa, and be ready to mate at appropriate times of the year if they are to have a fair chance of survival and of leaving descendants. The timing of these key events is under the influence of metabolic and neural activities which are believed to exhibit circadian (literally, about a day) and other rhythms.

These rhythms are synchronized by regular regimens of daylight and darkness, by regularly recurring high and low temperatures as can occur out-of-doors during daylight and darkness as night follows day. Other regularly recurring phenomena also may serve as synchronizers, and occasionally a single stimulus such as a light flash can synchronize a hatching or emergence rhythm. Responsiveness to these cues raises the odds that the weather and the availability of food or mates will be appropriate to the life stage of the insect.

Insect rhythms are subject to disruption if the regimen of the synchronizer is altered or disrupted. The usual result is an increased chance of death or a reduced chance of successful reproduction. Thus, rhythm modification might help 1) reduce those pesky face flies that suddenly appear when the days get longer, 2) enable large numbers of sterile lab-reared males to compete for mates in the field with small numbers of fertile wild males, 3) enable us to isolate natural hormones where the concentration is highest, 4) permit us to evaluate attractants when the insects will respond, 5) dictate when maximum response to treatments or insecticide application might be expected.

Chemical Desynchronization

We have been able to tinker with insects' bioclocks by altering light and temperature schedules; perhaps we can tinker biochemically as well with their time-measuring systems. Some chemicals are known which can shift the timing of the peak of some human rhythms; one of these is ACTH-1-17, a synthetic peptide similar to half the brain hormone molecule which stimulates the adrenal gland to produce steroids similar to cortisone. Such a peptide could be used to adjust the timing of the peak of some physiological function so that maximum effect of a treatment could be optimized for the benefit of the patient. Another use of such a material might be to prevent or minimize jet lag. In the case of pest insects, the rhythms would be desynchronized or the peaks would be shifted so that the insect would not complete its life cycle and so would not reproduce. Clearly, this would be a slower method of destroying the insect pest than exposure to a quick-acting stomach poison, but it might have a far less devastating impact on nontarget inhabitants of the ecosphere.

Diapause

On the other hand, the promotion of development rather than preventing growth may also be unfavorable to insect pests. Many insects have a built in mechanism, the diapause response. to keep them alive over periods of time in which food supplies are low or temperatures are not conducive to survival. Diapause in insects is like hibernation in bears; both insects and bears store fat, stop eating, and then use these fat reserves during hibernation at a much lower rate than if they were in the nondiapausing or hibernating state.

Some strains of a species will diapause and some will not. Genetic differences exist among these strains, but we do not understand the mechanisms fully. If an insect possesses the necessary genes, it will exhibit the diapause response. For diapause to occur, the environment must provide the appropriate signals, and the insect must be competent to respond. Interestingly enough, some strains of insects that have been maintained for a long time in the laboratory no longer diapause.

In diapause changes also may occur in the insect body fluids when compared to nondiapausing insects. An example is an increase in glycerol which apparently serves as a biological "antifreeze."

We are familiar with some but not all of the facts concerning how diapause comes about. For instance, we know that if the average temperature is above a threshold level during the time an insect is preparing to diapause, the insects will not diapause even though the photoperiodic regimen is appropriate. This critical temperature differs for each species and sometimes for the same species in each geographic area.

We know, too, that insect hormones which are required for maintaining the immature stage or for promoting development to the adult stage are at least quantitatively different in diapausing and nondiapausing individuals. When days are shortening in the fall, different quantities of hormones are being produced than in the longer summer days. Some scientists have suggested that a special diapause hormone is produced which, when secreted, promotes or maintains diapause.

Preventing Diapause

Diapause can be prevented by extending the natural day or by interjecting light breaks into the dark span in which insects are maintained This is true whether the insects are under field conditions or are domesticated in laboratory-rearing boxes. However, the biochemistry and physiology which must be investigated are not so clearly known.

The steroid hormone, ecdysone, which induces molting, or ecdysis, has a circadian cycle. Ecdysone levels in insect blood are high in insects synchronized in a day of 16 hours light-8 hours darkness and low in those held in 12 hours light-12 hours darkness. These data show just one measurement every 24 hours and, therefore, will not show the circadian phenomenon.

Precisely how the daylength is measured in insects is unknown. A peptide hormone may be involved. Humans are acquainted with large peptides such as insulin. In insects, molecules with a similar structure are involved. It is also unknown how exposure to appropriate temperature cycles can result in development or diapause if the organisms are maintained in constant darkness or constant dim light.

In about seven days of 16 hours of light and 8 hours of darkness, larva A develops pupal (worm-like) stage. Larva B, in 12 hours of light/12 hours of darkness sequence, molts several times but fails to develop pupa. (From the work of D. B. Gelman, ARS, USDA)

Manipulation of Light. Even without detailed knowledge of the biochemical steps, the principles of pest control by manipulation of daylight and darkness or photoperiod manipulation have been demonstrated. When we studied populations of European corn borer larvae in a stand of corn and codling moth larvae in apples out-of-doors, we changed the length of the day artificially, and the larval insects either developed into adults in the late, cold fall or the diapause state was disrupted over the winter so that spring emergence of sexually mature adults was prevented.

In this study, a span of 13 hours or less of daylight in the fall was extended to 16 hours with artificial lights. The light sources were either fluorescent lights in screen cages or large mercury arc lamps similar to those used to illuminate shopping center parking lots. This method of extension of the light span (or photo-phase) for preventing diapause was shown to be technically possible and might be an alternative to adding chemical pesticides to the environment. We and other investigators also have found that short interjections of light pulses prevented hibernation, but, in our hands, photophase extension produced more consistent results than did the light pulses.

At the time these tests were completed, it was evident that for a given application the costs for installing and operating fixed light sources to control insect pests would be at least 100-1,000 times those of commercial pesticides and other control measures for the average farm operator. Since changing daylight time with lights would be too expensive at the present time for farmers, other ways of tinkering with rhythms were studied.