Dennis J. Fielding

Developmental Time of Melanoplus sanguinipes (Orthoptera: Acrididae) at High Latitudes

Abstract Rates of development of Melanoplus sanguinipes F. from Alaska were determined at eight constant temperatures between 21 and 42°C. Diurnally alternating temperatures were used to estimate rates of development at temperatures too low for nymphs to complete development under constant temperatures. Two previously published equations were fit to these data and used to predict developmental rates as a function of temperature. The first equation, nonlinear and only approaching zero rate of development asymptotically, was fit to the data from constant and alternating temperature treatments. The second equation, which included an intercept, was fit to the data from constant temperature treatments only. Estimated developmental times based on these equations were tested against observed developmental times in two fluctuating temperature regimes in growth chambers, and two seasons of field sampling. Elevation of body temperature of grasshoppers above ambient temperatures in the field was modeled as a linear function of solar irradiance, based on field measurements. The effect of behavioral thermoregulation on nymphal developmental times in the field was estimated using standard air temperatures and solar-adjusted temperatures. Rates of development under most of the constant temperature treatments were higher than any previously published for M. sanguinipes. Estimated rates of development in the field using air temperatures only were about one-half those using solar-adjusted temperatures. Observed developmental times in the field were 45 and 42 d in 2000 and 2002, respectively. Because of local adaptation and behavioral thermoregulation, duration of nymphal stages in M. sanguinipes is relatively independent of latitude.

Growth, Development, and Nutritional Physiology of Grasshoppers from Subarctic and Temperate Regions

Despite the importance of developmental rate, growth rate, and size at maturity in the life history of poikliotherms, the trade‐offs among these traits and selection pressures involved in the evolution of these traits are not well understood. This study compared these traits in a grasshopper, Melanoplus sanguinipes F. (Orthoptera: Acrididae), from two contrasting geographical regions, subarctic Alaska and temperate Idaho. The growing season in the interior of Alaska is about 80 d shorter than at low‐elevation sites in Idaho. We hypothesized that the Alaskan grasshoppers would show more rapid growth and development than grasshoppers from Idaho, at the cost of greater sensitivity to food quality. On a diet of lettuce and wheat bran, grasshoppers from Alaska developed from egg hatch to adult more rapidly than those from Idaho at each of three different temperature regimes. Averaged over all temperature treatments, the weight of the Alaskan grasshoppers was about 5% less than that of the Idaho grasshoppers at the adult molt. Feeding and digestive efficiencies were determined for the final two instars using two meridic diets: one with a high concentration of nutrients and the other with the same formulation but diluted with cellulose. Alaskan grasshoppers again developed more rapidly, weighed less, and had faster growth rates than those from Idaho. Alaskan grasshoppers supported their more rapid growth by increasing postingestive efficiencies; that is, they had higher conversion rates of digested matter to biomass on the high‐quality diet, greater assimilation of food on the low‐quality diet, and greater efficiency of nitrogen assimilation or retention on both diets. There was no evidence that performance of Alaskan grasshoppers suffered any more than that of the Idaho grasshoppers on the low‐quality diet.

Spatial analysis of grasshopper density and ecological disturbance on southern Idaho rangeland

Abstract Historical grasshopper survey data were used to relate grasshopper densities to ecological condition classes on the Bureau of Land Managements Shoshone District in Idaho as part of an effort to identify areas prone to high grasshopper densities and to understand the effects of range management practices on grasshopper populations. A MOSS-based Geographic Information System was used to overlay maps of ecological condition, elevation, and grasshopper density to test the hypothesis that grasshopper density was the same for all ecological condition classes. Areas that had been severely disturbed by wildfires and invasion of exotic annual vegetation had significantly higher grasshopper densities over the 3 years examined than less severely disturbed area that retained some sagebrush cover. This suggests that shrub loss due to wildfires or other causes may lead to greater grasshopper densities. The techniques and applicability of geostatistical spatial analysis to these questions is also discussed.

Density and temperature-dependent melanization of fifth-instar Melanoplus sanguinipes: interpopulation comparisons

Abstract Behavioral thermoregulation, by basking in sunlight, is very common in acridids. The resulting increased body temperatures may be beneficial by accelerating feeding and developmental rates and by enhancing disease resistance. The absorption of solar energy is affected by the degree of cuticular melanization. In this paper, we quantify differences in color and thermoregulation between subarctic and temperate populations of Melanoplus sanguinipes. We also quantify effects of rearing temperatures and density on melanization of 5th instars. Alaskan grasshoppers tended to be darker than those from Idaho when reared under the same conditions and Alaskan grasshoppers also responded to cooler rearing temperatures by increased pigmentation. Density-dependent melanization was evident in both populations. Increased pigmentation enhanced the ability to thermoregulate. Absorption of wavelengths in the near infrared region was similar to that in the visible range, confirming that visible color is a reliable measure of relative absorption of total solar irradiance. Strong selection for efficient thermoregulation at high latitudes was suggested by the population differences in pigmentation.

Diapause Traits of Melanoplus sanguinipes and Melanoplus borealis (Orthoptera: Acrididae)

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 4–8 d to levels ≈70% lower than peak rates of prediapause, late stage embryos, in all populations. In the Idaho population, subjecting prediapause embryos to a cold treatment (5°C) 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 5°C 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 5°C. 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 flexibility in the diapause program among grasshoppers from Alaska.

Relationship of metabolic rate to body size in Orthoptera

Abstract Metabolic rate determines an individuals rate of resource acquisition, assimilation, growth, survival and reproduction. Studies involving a broad range of taxa and body sizes typically result in whole-organism metabolic rate scaling to the ¾ power of body mass. Competing models have been proposed to explain this allometric relationship. The nutrient supply network model of West et al. (1997) proposes that the ¾ power relationship results from the fractal nature of space-filling nutrient supply networks. The model of Kozlowski et al. (2003) proposes that the scaling of metabolic rate with body mass will vary from 2/3 to 1 among different taxa, depending on the degree to which increasing body size depends on increasing cell size or number. The present study measured resting metabolic rates across a broad range of body sizes in nymphs of Melanoplus sanguinipes F. and also analyzed published reports of metabolic rates in adult Orthoptera. The two sets of data were in close agreement: the scaling exponent for the ontogenetic series of M. sanguinipes was 0.92, and for the interspecific, phylogenetically corrected regression with adult Orthoptera, was 1.06. Both scaling exponents were significantly greater than the 0.75 predicted by the nutrient supply network model.

Populations of the northern grasshopper, Melanoplus borealis (Orthoptera: Acrididae), in Alaska are rarely food limited.

ABSTRACT In some systems, grasshoppers appear to be food limited in most years, whereas in other systems top down forces, for example, predators, are more often implicated in population regulation. Sustainable strategies to manage grasshopper populations through habitat management require knowledge of the forces that regulate grasshopper populations. This experiment was undertaken to determine whether populations of Melanoplus borealis (Fieber), a common pest species in Alaska, are food-limited in Alaska. Cages were set up in a fallow field near Delta Junction, AK, in 3 yr (2007–2009). In 2007 and 2008, fertilizer was added to half the plots to increase primary production, and, in all years, cages within each plot were stocked with 0, 5, 9, or 13 fourth-instar M. borealis (equivalent to 0, 20, 36, or 52 grasshoppers/m2). Grasshoppers in each cage were counted weekly. Near the end of the growing season, surviving female grasshoppers (≈40% of the original number) were collected. Femur length was taken as a measure of adult size, and functional ovarioles were counted as a measure of current fecundity. If the grasshoppers were food limited, we expected to see significant effects of either density or fertilizer on grasshopper survival, size, or fecundity. The fertilizer treatment greatly increased primary production in both years. Neither fertilizer treatment nor grasshopper density had consistent effects on survival, size, or potential fecundity, leading us to conclude that food is seldom limiting to populations in the interior of Alaska at densities <50 m-2.

Oviposition Site Selection by the Grasshoppers Melanoplus borealis and M. sanguinipes (Orthoptera: Acrididae)

Abstract Female grasshoppers can affect the fitness of their offspring through their selection of oviposition sites. Successful embryological development depends on suitable temperature and moisture levels, factors which may vary greatly on a fine scale in natural environments where grasshoppers occur. Knowledge of parameters defining acceptable oviposition sites can provide guidelines for habitat manipulations that reduce the availability of such sites. This paper compares oviposition-site selection in two species of grasshopper from Alaska, Melanoplus borealis and M. sanguinipes, and a population of M. sanguinipes from Idaho. Laboratory experiments did not detect any differences in preferred substrate temperature among the groups of grasshoppers. In field cages containing a choice of different ground covers (bare ground, single clump of grass or turf), M. sanguinipes from Idaho were less selective in terms of the distribution of egg pods among the different cover types, whereas both M. borealis and M. sanguinipes from Alaska deposited the fewest egg pods under the turf. Results suggest that differences in oviposition behavior between the grasshoppers from Idaho and those from Alaska may stem more from differences in the above-ground physical structure of the vegetation, than from differences in responses to below-ground temperature.

SPATIAL ANALYSIS OF GRASSHOPPER DENSITY AS INFLUENCED BY ANTHROPOGENIC HABITAT CHANGES

The rangeland environment in southern Idaho has been heavily impacted by human activities. Invasion by exotic plant species, frequent fires, grazing pressure, and other ecological disturbances have greatly affected the structure and dynamics of grasshopper populations. Quantification of spatial patterns of grasshopper density and species composition is important in order to determine their influence on grassland ecosystems, as well as evaluating managerial decisions concerning vegetation manipulations, grazing practices, and spraying programs. A spatial statistical approach to modeling the heterogeneity of grasshopper populations is presented, and the impact of vegetation and grazing treatments on grasshopper density is investigated. Empirical applications are demonstrated with reference to repeated field surveys conducted over several years in south central Idaho.