Stanley D. Beck

PHOTOPERIODIC INDUCTION OF DIAPAUSE IN AN INSECT

1. The European corn borer, Ostrinia nubilalis, is a so-called long-day insect, larval diapause being induced by naturally occurring photoperiods containing scotophases of from 10 to 14 hours.2. Diapause induction was found to be dependent upon the actual number of hours of the photoperiodic phases. The duration of the scotophase was far more critical than that of the photophase. A 12-hour scotophase was of maximum effectiveness when combined with photophases of from 5 to 18 hours. Significant incidence of diapause occurred when a 12-hour scotophase was combined with photophases of from 4.5 to 32 hours.3. Diapause induction is a temperature-sensitive phenomenon, with the incidence of diapause tending to be inversely proportional to the ambient temperatures occurring during the scotophase.4. Interruption of the scotophase by a one-hour period of light modified the photoperiodic response, the effect depending on the position of the light interruption within the scotophase. The effects were interpreted as a ...

Diapause in the European corn borer, Pyrausta nubilalis (Hübn.).

Abstract Diapause in mature larvae of the European corn borer was found to be associated with three readily discernible characteristics: (1) arrested gonadal development; (2) failure to pupate shortly after cessation of larval feeding; (3) reduction of oxygen consumption to about 25 per cent that of non-diapause mature borer larvae. Diapause was found to be induced by photoperiods of from 9 to 15·5 hr of light per 24-hr period. Within this range of photoperiods, incidence of diapause was inversely proportional to the rearing temperature employed, except that with 10·5–13·5 hr photoperiods all larvae entered diapause without regard to the ambient temperature. At moderate rearing temperature (23–25°C), the mean threshold photoperiod was 15·4 hr of light per 24 hr. The diapause threshold for photoperiods under 13 hr was not determined. The physiological changes associated with diapause were found to be largely reversible up to at least the early part of the fifth larval instar. No critical growth stage for diapause determination was detected.

Insect-Plant Interactions: Nutrition and Metabolism

From a phytochemical standpoint, plants are producers of chemicals, and insects are consumers. The biology of the consumer role played by insects is a very complex and intriguing subject, of which we will focus primarily on only two aspects — nutrition and metabolism. But even having so delimited the subject of discourse, it is immediately obvious that other aspects of insect-plant interaction must be considered, at least peripherally, for nutrition and metabolism are not isolated processes. They occur in conjunction with the behavioral and chemosensory facets of the biology of the phytophagous insects. It is an obvious truism that no insect is capable of utilizing every plant species, and conversely that no plant species is susceptible to attack by every species of plant-feeding insect. From both practical and theoretical viewpoints, the most important questions pertain to the identity of factors determining host specificity among the insects and susceptibility to insect depredation among the plants.

PHOTOPERIODIC TERMINATION OF DIAPAUSE IN AN INSECT

1. The European corn borer, Ostrinia nubilalis, has a faculative diapause in the last larval instar. Diapause induced in the laboratory by a short-day photoperiod is identical in its intensity to that occurring in the field.2. Diapause development occurs at 30° C. under various photoperiodic conditions but is greatly accelerated by a long day.3. Completion of diapause development does not require a period of chilling.4. Diapause development is a process that removes a block to secretion of the activation hormone but does not include secretion or any of the morphogenic events that follow.

Intracellular symbiotes of the pea aphid, Acyrthosiphon pisum☆

Histological and ultrastructural studies on the mycetome of the pea aphid, Acyrthosiphon pisum, disclosed two types of symbiotes. The more common primary symbiotes were oval in shape and were found in large mycetocytes making up the bulk of the mycetome. The secondary symbiotes were smaller, rod-shaped, and were restricted to an apparently syncytial sheath partially enclosing the primary mycetocytes. Extensive rough endoplasmic reticulum and Golgi occurred in the sheath but not in the primary mycetocytes. Lysosomal breakdown occurred in both primary and secondary symbiotes but the two processes differed markedly. In the primary mycetocytes, a small number of symbiotes were broken down individually to form small, compact residual bodies. In the sheath, breakdown of secondary symbiotes was more extensive: large numbers were broken down within cytolysomes.

Nutrition of the pea aphid, Acyrthosiphon pisum: requirements for trace metals, sulphur, and cholesterol

Trace metals, inorganic sulphur, and cholesterol benzoate were investigated in diets for the pea aphid, Acyrthosiphon pisum. The aphids were reared on sterile holidic diets for more than 46 generations (123years), without decline in weight or fecundity. Aphid performance was severely limited by deficiencies of Fe3+ within 1 generation and within 2 generations for Cu2+. Higher levels of trace metals were required for optimal reproduction than for growth. The optimal levels of trace metals as cations per 100 ml of diet were found to be: Fe3+, 920 μg; Zn2+, 400 μg; Cu2+, 120 μg; and Mn2+, 220 μg. Twenty-eight generations of aphids were supported on a sterol-free diet. However, aphid performance was better on diets that contained both sterol and inorganic sulphur at the highest levels tested (2·5 and 32mg100ml of diet, respectively) than on sterol-free diets with or without a supplement of inorganic sulphur. Pea aphids reared on the best diet tested had a mean adult weight of 2·06 mg, which was 88 per cent of that of aphids reared on the broad bean plant, Vicia faba L. The average number of progeny per diet-reared aphid per day was 2·2 compared to a mean of 5·5 per day among plant-reared aphids.

Interrelationships of nutritional indices and dietary moisture in the black cutworm (Agrotis ipsilon) digestive efficiency

A study was made of the relationships among the physiological functions measured in nutritional index experiments using the black cutworm (Agrotis ipsilon). Some of the relationships were expected: e.g. larvae that weighed more at the beginning of the experimental period ate more, gained more weight, and produced larger quantities of faeces. Significantly, no interactions were found between initial larval weight and assimilation, efficiency of conversion of assimilated food, or efficiency of conversion of ingested food. Experiments in which the dietary moisture level was varied showed that a dilution was not compensated for by increased ingestion. Very dry diets resulted in a decrease in fresh weight eaten, as well as in dry weight eaten. As has been reported by other investigators, efficiency of conversion of assimilated food and efficiency of conversion of ingested food were negatively correlated with percent dry matter of the diet. Growth clearly depends both upon the amount of diet (dry weight) ingested and upon efficiency of conversion of ingested food. Accordingly, in our experiments, growth decreased somewhat on the low percent dry matter diets, and decreased a great deal as percent dry matter increased above the control diet level. Optimal growth occurred at a percent dry matter level very close to that of the control diet level, but below that for maximal dry weight eaten, because of the influence exerted by the efficiency of conversion of ingested food. Percent dry matter of faeces increased as percent dry matter of diet increased in a remarkably precise fashion. Percent dry matter of the larvae seemed to be under little regulation, although the larvae were capable of some water conservation. The implications of these findings on the physiological ecology of black cutworm larvae are discussed.

Haemolymph proteins of Ostrinia nubilalis during diapause and prepupal differentiation

Metabolism during diapause and prepupal differentiation of the European corn borer, Ostrinia nubilalis (Hubner), was evaluated on the basis of changes in fat body proteins. The protein concentration results showed that, although the prepupal titre of about 75 mg/g dry fat body remained fairly constant throughout diapause, an increase and sex difference were detected in postdiapause pharate pupae. In these latter forms the males had a protein titre of 253 ± 14 mg/g dry fat body as compared to the 393 ± 20 mg/g titre of the females. The disk electrophoretic results showed that 10- to 14-day-old larvae had protein patterns composed of about ten anionically migrating bands. By the fifteenth day, however, the larvae were reaching maturity, and a sex difference was apparent. The females developed a high titre of another protein, Rf 13, which was not detectable in males. To determine whether changes occurred during diapause, fat body proteins from short-day and long-day exposed prepupae were compared. The pattern data indicated that, in both sexes, only minor changes occurred during this period. Dramatic changes were not detected until the pupal moulting cycle, characterized by the existence of only two major bands (Rf 12 and 19) in the pharate pupae. Circumstantial evidence suggested that in the female one of these bands (Rf 12) eventually was translocated to the ovary. A study of glucose-6-phosphate and malic dehydrogenase activity of prepupal fat body revealed one and two major bands, respectively, without an ostensible sex difference. These bands appeared in both short-day and long-day exposed prepupae, showing no apparent changes during diapause. The implications of the results are discussed in relation to the metabolic role of the fat body during this period of metamorphosis.

Cold hardiness in the European corn borer, Pyrausta nubilalis (Hübn.)☆

Abstract The relationships between undercooling capacity, tissue freezing, and survival at subzero temperatures were studied in mature larvae of the European corn borer, Pyrausta nubilalis (Hubn.). Undercooling temperatures were determined by means of a thermistor-thermometer. Larval survival was measured at −10, −15, and −20°C. It was found that the undercooling temperatures of the larvae were not a reliable indication of their ability to survive under subzero conditions. Cold-hardening of borer larvae in the field began in August and continued to increase until late November. Summer larvae in diapause succumbed rapidly to low temperatures. Cold-hardy winter larvae, on the other hand, survived up to 3 months at −20°C, when in the absence of contact moisture. Chilling in the presence of contact moisture caused freezing of the larvae and reduced their ability to survive cold conditions. Despite the formation of ice in their tissues, fully cold-hardy borers survived several weeks while frozen at −20°C. The undercooling temperatures of cold-susceptible larvae were lowered by chilling and desiccation, but such treatment did not significantly enhance their ability to survive subzero conditions. Cold hardiness was destroyed by incubating cold-hardy larvae at 30°C, either with or without contact moisture.

Thermoperiodic induction of larval diapause in the European corn borer, Ostrinia nubilalis

Abstract Thermoperiodic induction of larval diapause was shown to occur in the European corn borer, Ostrinia nubilalis (Hubner). Using continuous darkness and thermoperiods of the format XC:24-XT(15:30°C), incidence of diapause was shown to be dependent on phase durations, with a critical cryophase of about 9.5 hr. A thermoperiodic response threshold was demonstrated; it was found to be very close to 17.5°C. Thermoperiodic amplitude (temperature difference between cryophase and thermophase) was shown to have no influence on the induction of diapause, within a relatively broad range of physiological temperatures. Thermoperiodic induction of diapause was shown to be at least partially independent from the effects of temperature on larval developmental rates.