Culture and Colonization
L.K. ETZEL, E.F. LEGNER, in Handbook of Biological Control, 1999
Diapause
Diapause in the life cycle often interferes with parasitoid and predator production. For example, Eskafi and Legner (1974) showed that certain temperature and photoperiod combinations would induce diapause in adults and progeny of the eye gnat parasitoid Hexacola sp. nr. websteri (Crawford). Exposing larval parasitoids within their larval hosts to a long photophase of 16 h combined with a high temperature of 32°C caused the parasitoid prepupae to enter a diapause state, termination of which required contact of the host puparia with moisture for a few hours. However, this type of easily terminated diapause only occurred following a parental generation reared at 27°C with 14-h light. With the parasitoid parental generation reared at 32°C with 16-h light and the progeny held at 27°C with 14-h light, then >90% of the prepupal progeny entered diapause and could not be induced to terminate it by exposure to moisture. With another set of progeny from the same parents reared at 32°C with 16-h light, only 35% entered diapause. This example illustrates the great complexities involved in determining which combinations of environmental regimes in the insectary will prevent, induce, or terminate diapause.
In the case of Trichogramma spp., Zaslavski and Umarova (1990) showed that the interactive effects of environmental conditions in the maternal and filial generations governed larval diapause in the filial generation. Lowered temperature during development of the filial generation was the predominant diapause factor, but the photoperiod and temperature conditions of the maternal generation influenced the “norm” of this thermal reaction. Superimposed on these diapause reactions was an endogenous process running through the generations that changed diapause tendency and underlying reaction norms even under constant rearing conditions (Zaslavski & Umarova, 1990). Other examples of complex diapauses include the alfalfa weevil parasitoid system; Chelonus spp. parasitoids of the pink bollworm that terminate diapause at different intervals (Legner 1979c); and navel-orangeworm parasitoids, where diapause seems triggered by hormonal changes in the host at different latitudes (Legner, 1983).
Not only physical environmental conditions but also host conditions and host food can influence diapause in parasitoids. The physiological state of an alfalfa plant affects the yellow clover aphid (formerly called the spotted alfalfa aphid), Therioaphis trifolii (Monell), which then induces diapause in its aphidiid parasitoid Praon exsoletum Nees (Clausen, 1977). Similarly, the type of pollen used to rear predaceous mites can affect initiation of diapause. Amblyseius potentillae (Garman) and A. cucumeris entered diapause in a short-day photoperiodic regime when reared on ice plant pollen, but not when reared on pollen of the broad bean (Overmeer et al., 1989). After studying various combinations of parasitoid species, aphid species, plants, and environmental conditions, Polgar et al. (1995) concluded that internal factors via the host aphid, such as host aphid life cycle (holocyclic versus anholocyclic), aphid morph (oviparae, etc.), and host-plant quality, as well as environmental cues (temperature and photoperiod), can all be interconnected in the induction of dormancy or diapause in aphid parasitoids.
Laing and Corrigan (1995) showed the importance of host species in initiating diapause in Trichogramma minutum. Diapause occurred at 15°C with a photophase of 12L:12D in eggs of Lambdina fiscellaria fiscellaria, but did not occur in eggs of Anagasta kuehniella, Sitotroga cerealella, or Choristoneura fumiferana held under these same conditions.
Appropriate environmental conditions or their combinations, particularly relating to light and temperature, are often useful for manipulating diapause (Singh & Ashby, 1985). For example, Tauber et al. (1997b) showed that a biotype of Chrysoperla carnea from San Pedro, Mexico, could be reared continuously without diapause with an intragenerational increase in photoperiod or could be reared with the regular intervention of a storage period. Diapause induction in individuals destined for storage was accomplished by rearing larvae under a long-day photoperiod and transferring the prepupae to a short-day photoperiod.
Waage et al. (1985) noted that one factor to consider in rearing programs is that entomophagous insects and their hosts may have different optimal developmental temperatures. As a corollary, natural enemies may enter diapause under conditions whereby their hosts remain active. The western flower thrips, Frankliniella occidentalis (Pergande), is a year-round greenhouse pest in northern climates, whereas commercially available strains of two predaceous mites, Amblyseius cucumeris and A. barkeri (Hughes), are ineffective in the winter because of reproductive diapause induced by short-light conditions. Van Houten et al. (1995), however, were able to genetically select for effective nondiapausing strains of these predators within 10 laboratory generations.