Journal of Economic Entomology (2007) 100, 627-645

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Toomas Esperk, Toomas Tammaru and Sören Nylin (2007)
Intraspecific variability in number of larval instars in insects
Journal of Economic Entomology 100 (3), 627-645
Abstract: 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.
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Research topic(s) for pests/diseases/weeds:
general biology - morphology - evolution

Pest and/or beneficial records:

Beneficial Pest/Disease/Weed Crop/Product Country Quarant.

Tineola bisselliella
Sitophilus oryzae
Oryzaephilus surinamensis
Tenebrio molitor
Trogoderma glabrum
Trogoderma variabile
Xestia c-nigrum
Helicoverpa armigera
Lymantria dispar
Simulium vittatum
Schistocerca gregaria
Chilo partellus
Choristoneura fumiferana
Choristoneura occidentalis
Trichoplusia ni
Chloridea virescens
Anticarsia gemmatalis
Chrysodeixis includens
Diatraea grandiosella
Pieris brassicae
Spodoptera litura
Spodoptera littoralis
Spodoptera frugiperda
Diatraea saccharalis
Diatraea lineolata
Cosmopolites sordidus
Zonocerus variegatus
Sesamia nonagrioides
Manduca sexta
Autographa gamma
Dermestes lardarius
Attagenus unicolor
Attagenus fasciatus
Pectinophora gossypiella
Nomadacris septemfasciata
Mythimna separata
Tabanus nipponicus
Trogoderma inclusum
Lagria hirta
Gryllus bimaculatus
Omocestus viridulus
Anthrenus coloratus
Tabanus lineola