Cold hardiness and diapause are both essential components of winter survival for most insects of the temperate zone. But, in many cases, it is not clear how these two are related. Are they independent events or is cold hardiness a component of the diapause syndrome? Both independence (Lees, 1955; Salt, 1961; Ring, 1972) and dependence (Asahina, 1969; Mansingh, 1971, 1974) of cold hardiness and diapause have been defended vigorously, and indeed evidence for both possibilities can be found in the literature. In this chapter I argue that cold hardiness can be achieved independently of diapause, but cold hardiness is often a component of the diapause syndrome and the expression of diapause frequently extends the insect’s capacity to cold harden.

Relationship between Cold Hardiness and Diapause

The number of larval instars varies widely across insect species. Although instar number is frequently considered to be invariable within species, intraspecific variability in the number of instars is not an exceptional phenomenon. However, the knowledge has remained fragmentary, and there are no recent attempts to synthesize the results of relevant studies. Based on published case studies, we show that intraspecific variability in the number of larval instars is widespread across insect taxa, occurring in most major orders, in both hemimetabolous and holometabolous insects. We give an overview of various factors that have been observed to affect the number of instars. Temperature, photoperiod, food quality and quantity, humidity, rearing density, physical condition, inheritance, and sex are the most common factors influencing the number of instars. We discuss adaptive scenarios that may provide ultimate explanations for the plasticity in instar number. The data available largely support the compensation scenario, according to which instar number increases in adverse conditions when larvae fail to reach a species-specific threshold size for metamorphosis. However, in Orthoptera and Coleoptera, there are some exceptional species in which instar number is higher in favorable conditions. In more specific cases, the adaptive value of the variability in instar number may be in reaching or maintaining the developmental stage adapted to hibernation, producing additional generations in multivoltine species, or increasing the probability of surviving in long-lasting adverse conditions.

Intraspecific Variability in Number of Larval Instars in Insects

Insect Nutrition: An Adaptationist's Perspective

Larval instar as a key element of insect growth schedules

SUMMARY Repeated freeze–thaw cycles are common and are increasing in frequency with climate change in many temperate locations, yet understanding of their impact on freeze-tolerant insects is extremely limited. We investigated the effects of repeated freezing and thawing on the freeze-tolerant final instar caterpillars of the moth Pyrrharctia isabella (Lepidoptera: Arctiidae) by subjecting individuals to either a single sustained 35 h freeze or five 7 h freezes. Sub-lethal effects were quantified with changes in three broad groups of measures: (1) cold hardiness, (2) metabolic rate and energy reserves and (3) survival after challenge with fungal spores. Repeated freeze–thaw cycles increased mortality to almost 30% and increased tissue damage in Malpighian tubules and hemocytes. Repeated freezing increased caterpillar glycerol concentration by 0.82 mol l –1 . There were no changes in metabolic rate or energy reserves with repeated freezing. For the first time, we report increased survival after immune challenge in caterpillars after freezing and suggest that this may be linked to wounding during freezing. We suggest that little repair of freezing damage is possible in P. isabella caterpillars and repeated freeze–thaw cycles may present significant challenges to survival in this species.

/pdf/the-sub-lethal-effects-of-repeated-freezing-in-the-woolly-2x3lqhibyf.pdf

The sub-lethal effects of repeated freezing in the woolly bear caterpillar Pyrrharctia isabella

Effects of photoperiod and temperature on the development of a univoltine population of the banded woollybear, Pyrrharctia (Isia) isabella

Head capsule widths of laboratory-reared and mature field-collected larvae of P. isabella were measured. The number of larval instars varied from 7 to 10, yet in all larvae studied, average size and increment in the successive instars were the same for instars 1 to 3. In the latter instars, there was an inverse relationship between size of increment and prospective number of instars. Insects which completed 10 instars were only a little larger than those maturing after only 7 instars. These data fail to support Dyar’s rule.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0008347X00012657

Further studies on the biology of pyrrharctia (isia) isabella (lepidoptera: arctiidae): iii. the relation between head capsule width and number of instars

The banded woollybear, Pyrrharctia (Isia) isabella (J. E. Smith,) was reared under laboratory conditions from egg to adult. Individually reared larvae were fed one of six artificial diets and fresh plantain at 26 °C, light/dark (h) 15/9. The times of moulting, larval and pupal weights, and adult characteristics (fertility, sex) were recorded.Larvae were easily reared on bean, wheat germ, casein – wheat germ and fresh plantain, while casein, alfalfa, and plantain diets proved unsatisfactory. Rearing procedure and developmental characteristics are discussed.

Laboratory rearing of the banded woollybear, pyrrharctia (isia) isabella (lepidoptera: arctiidae), on different diets with notes on the biology of the species

The banded woollybear, Pyrrharctia (Isia) isabella (J. E. Smith), was reared from egg to larval diapause at 22, 26, and 30 °C. Field-collected and laboratory-reared larvae were weighed twice weekly and kept at different temperatures (0, 5, 10, 22, 26, and 30 °C). The larvae reared at 22 and 26 °C were found to remain viable at 5 °C, whereas those reared at 30 °C were not. Chilling was not an absolute requirement for diapause development, although mortality was high at higher temperatures. Temperature is critical in the maintenance of weight and for survival of diapausing larvae. Other factors than diapause seem to be involved in cold hardening.

Further studies on the biology of the banded woollybear, Pyrrharctia (Isia) isabella (J. E. Smith) (Lepidoptera: Arctiidae). IV. Diapause development as influenced by temperature

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Effects of photoperiod and temperature on the development of a univoltine population of the banded woollybear, Pyrrharctia (Isia) isabella

Further studies on the biology of pyrrharctia (isia) isabella (lepidoptera: arctiidae): iii. the relation between head capsule width and number of instars

Laboratory rearing of the banded woollybear, pyrrharctia (isia) isabella (lepidoptera: arctiidae), on different diets with notes on the biology of the species

Further studies on the biology of the banded woollybear, Pyrrharctia (Isia) isabella (J. E. Smith) (Lepidoptera: Arctiidae). IV. Diapause development as influenced by temperature