PHYSIOLOGY,BIOCHEMISTRY, AND TOXICOLOGY Diapause Traits of Melanoplus sanguinipes and Melanoplus borealis (Orthoptera: Acrididae) DENNIS J. FIELDING1 USDAÐARS, P.O. Box 757200, University of Alaska, Fairbanks, AK 99775 Ann. Entomol. Soc. Am. 101(2): 439Ð448 (2008) ABSTRACT A thorough understanding of diapause development in insects is needed for prediction of population responses to climate change, for realistic simulation models, and for effective pest management. In Melanoplus sanguinipes F. and Melanoplus borealis (Fieber) (Orthoptera: Acrididae), diapause typically occurs in embryos at a late stage of development. Experiments were conducted to compare diapause traits in two populations of M. sanguinipes, one from Alaska and one from Idaho, and also in a population of M. borealis from Alaska. Respiration was measured at different stages of embryonic development. As the embryos entered diapause, respiration rates declined over a period of 4Ð8 d to levels Ϸ70% lower than peak rates of prediapause, late stage embryos, in all populations. In the Idaho population, subjecting prediapause embryos to a cold treatment (5ЊC) caused the embryo to avert diapause: respiration in these embryos continued to increase after chilling as the embryo developed directly to hatching. In M. borealis and M. sanguinipes from Alaska, chilling of prediapause embryos did not affect subsequent diapause development: respiration rates in these embryos increased until they entered diapause and then declined. About the same amount of time at 5ЊC was necessary to avert diapause in early stage embryos of the Idaho population as was required to terminate diapause in late stage embryos. Respiration rates of diapausing embryos increased with increasing time spent at 5ЊC. Respiration continued to increase after transfer to warm temperatures in those that had completed diapause, but in those that had not, respiration soon declined back to diapause levels. In general, there was less ßexibility in the diapause program among grasshoppers from Alaska. KEY WORDS diapause, Melanoplus, chilling, respiration Diapause in insects is characterized by cessation of predict spread of invasive species and to predict the morphological development, even though tempera- response of insects to climate change (Bradshaw and ture and other environmental conditions may be fa- Holzapfel 2001, Regniere and Nealis 2002, Olfert et al. vorable for continued development. Diapause enables 2004). insects to synchronize growth and reproduction to Acridids are no exception when it comes to the predictable, seasonal changes in environment importance of diapause in their life cycle. From trop- (Tauber et al. 1986). Traits related to diapause, e.g., ical to subarctic climates, an understanding of the the stage at which it occurs, whether obligate or fac- diapause process is vital to understanding and pre- ultative, its duration, are tailored by natural selection dicting population dynamics of grasshoppers and lo- to favor adaptation of the organism to its environment custs (Valek and Coppel 1972, Cherrill and Begon (Tauber et al. 1986, Bradshaw et al. 2000, Fielding 1991, Groeters 1994, Tanaka 1994, Gehrken and 2006). Diapause may inßuence population dynamics Doumbia 1996, Hunter et al. 2001, Fielding 2006). by controlling voltinism (Steinbauer et al. 2004) and Different species of acridids may diapause at different by affecting interactions between parasitoids and pathogens and their hosts (Ringel et al. 1998, Corley stages, but most common is the egg stage. Among those et al. 2004). Effective pest management, including species that diapause as embryos there is considerable management of resistance to insecticides, relies on an variation in the different aspects of diapause, such as understanding of diapause attributes (Carrie`re et al. the stage of embryonic development at which dia- 1995, Mitchell and Onstad 2005), and models of insect pause occurs, whether diapause is facultative or obli- populations often require realistic simulation of the gate, and the time required to complete diapause diapause process (Gage et al. 1976, Carter et al. 1998, (Moore 1948, Kreasky 1960, Gage et al. 1976, Tanaka Onstad and Gould 1998). Furthermore, a thorough 1994). There also may be considerable variation understanding of the diapause process is necessary to among populations within a species in characteristics related to diapause (Dingle et al. 1990, Dingle and 1 Corresponding author, e-mail: [email protected]. Mousseau 1994, Groeters 1994). 440 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 101, no. 2 Melanoplus sanguinipes F. has a wide geographic different stages of development to determine the stage distribution, from central Florida and Mexico to the at which diapause occurs, whether diapause could be interior of Alaska (Capinera et al. 2002, Pfadt 2002), averted by chilling of prediapause embryos, and the and it exhibits a correspondingly diverse range of amount of time at cool temperatures required to ter- diapause characteristics. This species overwinters in minate or avert diapause. the egg stage only, but the embryos do not need to be in diapause to survive the winter. In southern portions Materials and Methods of their range, populations are bivoltine and embry- onic diapause may be under maternal control, in re- Laboratory colonies of M. sanguinipes from Idaho sponse to changing photoperiod (Dean 1982). In more were initiated with at least 200 individuals collected northern regions where populations are univoltine, near Lewiston, ID (46.38Њ N, 117.02Њ W, 450-m eleva- diapause prevents the eggs from hatching too late in tion) as fourth and Þfth instars in 2003 and 2004. the season to complete another generation. In the far Laboratory colonies of M. sanguinipes and M. borealis north, eggs of semivoltine populations overwinter from Alaska were initiated with similar numbers of twice, but not because of an extended diapause or insects collected near Delta Junction, AK (64.00Њ N, because they diapause twice. Reproduction in Alaska 145.73Њ W, 400-m elevation). Grasshoppers were main- begins in late summer. Soil temperature soon declines tained on romaine lettuce and wheat bran, at room below the threshold for development, and embryos temperature with incandescent lights (16-h photo- spend the Þrst winter in an early stage of development phase) suspended above the cages to allow the grass- (D.J.F., unpublished data). These eggs are not in dia- hoppers to self-regulate their internal temperatures. pause when overwintering for the Þrst time. The next Trays of moist sand were provided for oviposition. summer, embryos develop further until entering dia- Offspring from the Þeld-collected grasshoppers were pause. Diapause is terminated sometime before mor- reared under the same conditions described above, phological development resumes the next spring and and all experiments were conducted on F2 and F3 the eggs hatch. generation eggs to avoid maternal effects from the Most populations of M. sanguinipes diapause in a late Þeld-collected generation. Oviposition trays were stage of development, Ϸ80% of development from sifted daily to obtain egg pods of known age. Egg pods oviposition to hatch (Moore 1948, Dingle et al. 1990). were stored in moistened vermiculite in plastic cups Diapause at a late stage of development allows for a (50 ml) with perforated lids. relatively early hatch date because less development Hatching in Relation to Developmental Stage and remains to be completed before hatching in the spring. Chill Time. To determine the effect of stage of de- Dingle and Mousseau (1994) reported variation in the velopment and duration of chilling on diapause avoid- diapause stage within and among populations of M. ance or termination, eggs of different ages were sanguinipes along an altitudinal gradient, with some chilled for different periods of time and subsequent diapausing before katatrepsis. development at higher temperatures was observed. Previous investigations suggested that diapause was Katatrepsis (reorientation of the embryo within the obligate in an Alaskan population of M. sanguinipes egg, also referred to as blastokinesis Chapman 1982), (Fielding 2006). If chilled at early, prediapause stages was used as a benchmark to determine stages of em- of development (simulating overwintering for the Þrst bryological development. Katatrepsis in Melanoplus time) and then returned to warm temperatures, the spp. takes place shortly after the temporal midpoint of embryos continued development until diapause was embryological development (Moore 1948). Eggs were initiated and then would not hatch unless diapause held at 22 Ϯ 1ЊC until transfer to a refrigerator main- was terminated by chilling a second time. By contrast, tained at 5 Ϯ 1ЊC. Egg pods were transferred to the eggs of the same species from Idaho hatched after cool temperature at three different stages of devel- experiencing cool temperatures at any stage of devel- opment: before katatrepsis, after katatrepsis but be- opment. It seemed that embryos of the Idaho popu- fore diapause, and after diapause was initiated. At lation averted, or avoided, diapause, as opposed to 22ЊC. These stages of development correspond to ages entering diapause at an early stage of development. of 12Ð17 d, 22Ð27 d, and Ͼ36 d for both species of Church and Salt (1952) alluded to diapause avoidance Alaskan grasshoppers. Three to Þve eggs from each in Melanoplus bivittatus (Say), and Cherrill and Begon pod were examined to conÞrm the embryosÕ stage of (1991) describe