Kevin M. O'Neill

Patterns of vegetation and grasshopper community composition

SummaryA study was conducted to evaluate differences in rangeland grasshopper communities over environmental gradients in Gallatin Valley, Montana, USA. The concept of habitat type (Daubenmire 1966) was used as a basis for discriminating between groupings of patches based on vegetation. A total of 39 patches were selected that represented five recognized grassland habitat types (Mueggler and Stewart 1980), as well as two disturbed types (replanting within a known habitat type). Repeated sampling in 1988 of both the insect and plant communities yielded a total of 40 grasshopper (19 664 individuals) and 97 plant species. Detrended Correspondence Analysis (DCA) indicated that patch classifications based on presence and percent cover of plants were appropriate and showed good between-group (habitat type) separation for patches along gradients of precipitation/elevation and plant community complexity. Results from undisturbed habitats showed that plant and grasshopper species composition changed over observed environmental gradients and suggested that habitat type influenced not only species presence, but also relative abundance. Discussion is presented that relates results with patch-use and core and satellite species paradigms.

Effects of livestock grazing on grasshopper abundance on a native rangeland in Montana.

ABSTRACT Livestock grazing can affect habitat quality for grasshoppers through effects on food and oviposition site availability, microclimate, and other factors. Because of this, some authors have suggested that grazing programs can be used to help manage pest grasshopper populations. In a 6-yr study, we controlled access of cattle to replicated experimental plots on an Agropyron spicatum/Poa sandbergii pasture to create consistent year-to-year differences in postgrazing plant cover, with resultant affects on microclimate. After sampling grasshoppers multiple times after grazing treatments each summer, we found evidence of between-treatment differences in grasshopper abundance for the entire assemblage during 4 of the 6 yr. Some species, including Melanoplus sanguinipes (perhaps the worse rangeland grasshopper pest in the western United States), tended to be more abundant on ungrazed plots, whereas Melanoplus gladstoni often had greater densities on heavily-grazed plots. The effect of grazing on grasshopper densities in this study was lower in magnitude and less consistent among years than in a study we conducted simultaneously at a nearby site where the vegetation was dominated by the exotic species crested wheatgrass (Agropyron cristatum). Our results generally support proposals that grazing could be used to reduce pest grasshopper densities, although the effectiveness of a particular grazing scheme may vary among sites, years, and grasshopper and vegetation assemblages.

Offspring Size and Sex Ratio Variation in a Feral Population of Alfalfa Leafcutting Bees (Hymenoptera: Megachilidae)

ABSTRACT The alfalfa leafcutting bee, Megachile rotundata (F.) (Hymenoptera: Megachilidae), is widely used in western North America to enhance pollination of alfalfa, Medicago sativa L., grown for seed production. Feral populations have been documented but have been little-studied, We used trap-nests of a wide range of diameters (3–9 mm) to study a feral population at a wildflower seed farm in Montana. Adult females accepted all hole sizes but tended to provision nest tunnels of smaller diameters (3–4 mm) than those provided in managed populations (5–7 mm). Progeny, especially females, from intermediate- and large-diameter nests averaged larger body size that those in the two smallest nest-diameter classes. Offspring were often larger in nests with greater numbers of offspring, indicating that there was no trade-off between offspring size and number within nests. Individual nests tended to contain a relatively small range of progeny sizes and those sizes tended to vary among nests of the same diameter. Previous studies of M. rotundata indicate that heritability of body size is low, most offspring within nests are full siblings, and variation in offspring size is due to variation in the amount of provision provided by mothers. Thus, the fact that body size varied among families probably related to variation in provisioning capabilities among females. We also found evidence that offspring size varied spatially but that seasonal trends in offspring size were weak. Sex ratio varied among different nest sizes, but only the smallest and largest nests produced biased ratios. Sex ratios were less male-biased than in commercially managed populations of M. rotundata.

Effect of Temperature on Post-Wintering Development and Total Lipid Content of Alfalfa Leafcutting Bees

ABSTRACT Temperature plays an important role in effective management of the alfalfa leafcutting bee [Megachile rotundata (F.); Megachilidae], the major commercial pollinator of seed alfalfa [Medicago sativa (L.); Fabaceae] in North America. To improve our understanding of threshold and optimum rearing temperatures of M. rotundata, we examined the effect of temperature on postwintering development by using a greater number of temperature treatments than applied in previous studies (19 versus eight or fewer) and analytical tools formulated to model nonlinear relationships between temperature and insect development rates. We also tested the hypothesis that rearing temperature influences adult body lipid content at emergence, which could affect adult survival, establishment and performance as a pollinator, and reproductive success. We found that the Lactin-2 and Briere-2 models provided the best fits to data and gave reasonable estimates of lower (16–18°C) and upper (36–39°C) developmental thresholds and optimum (33–34°C) rearing temperatures for maximizing development rate. Bees successfully emerged over a broad range of temperatures (22– 35°C), but variation in development rate among individuals reared at the same temperature was lowest at 31–33°C. The optimum rearing temperature to maximize the proportion of body lipids in adults was 27–29°C. Our results are discussed in relation to previous findings and speak to the difficulties in designing practical rearing guidelines that simultaneously maximize development rate, survival, and adult condition, while synchronizing adult emergence with alfalfa bloom.

Prey, Nest Associates, and Sex Ratios of Isodontia mexicana (Saussure) (Hymenoptera: Sphecidae) from Two Sites in New York State

Abstract At two locations in central New York, including the Montezuma National Wildlife Refuge, Isodontia mexicana females nested in 8 and 9 mm diameter trap nests. As is typical for Isodontia, partitions and plugs in the nests were constructed using fragments of grass stems, and the outermost portion of the final nest plug consisted of a tuft of grass leaves that extended as far as 7 cm beyond the nest entrance. The vast majority of prey were adult tree crickets (Gryllidae: Oecanthinae; Oecanthus), although nymphal tree crickets and katydids (Tettigoniidae: Conocephalinae) also occurred among provisions. The sex ratio of adult tree cricket prey was strongly female-biased. Nest associates included three other nest-provisioning aculeates, along with Psocoptera, spiders, ants, Coelioxys sp. (Megachilidae), and bombyliid, sarcophagid, and phorid flies. The overall sex ratio of I. mexicana offspring did not differ from unity, but the sex of offspring in any given cell was related to the position of the cell and the total number of cells in a nest. As females added cells to the linear sequence in each nest, new cells were increasingly likely to house sons, especially in nests with fewer cells. Surprisingly, given results of previous studies, the sex of an offspring was unrelated to nest tunnel diameter.

Short-term dynamics of behavioral thermoregulation by adults of the grasshopper Melanoplus sanguinipes.

Abstract The short-term behavioral responses of adult grasshoppers, Melanoplus sanguinipes (F.) (Orthoptera: Acrididae), were examined after they experienced changes in microclimate when beingforced to change positions in their habitat. It was also determined if and when behavioral tactics allowed adults to achieve body temperatures within their preferred range. The preferred or set-point range, here taken as the interquartile range of temperatures selected on a laboratory thermal gradient, was estimated to be 37.4–40.5°C. In the field, adults progressed through a relatively consistent daily sequence of behaviors, basking on the soil early in the day, but moving onto vegetation as temperatures increased. Although basking allowed grasshoppers to maximize body temperature within the available range, as much as 7°C in excess of air temperature, they could not attain preferred body temperatures until soil surface temperatures reach about 35°C. Basking was more effective in grazed than ungrazed pastures due to a lower degree of shading of the soil surface. As soil surface temperatures exceeded 35°C, grasshoppers could achieve body temperatures within the preferred range by moving to the appropriate height on vegetation. These results illustrate the advantage of assessing behavior in the field in relation to preferred body temperatures determined in the laboratory.

WORKER RESPONSE TO THERMAL CONSTRAINTS IN THE ANT FORMICA OBSCURIPES (HYMENOPTERA: FORMICIDAE)

Abstract 1. 1.In southwestern Montana, the ant Formica obscuripes Forel occupies open habitats where temperatures on soil and nest surfaces of 50–60°C are common in summer. Workers succumb rapidly if they remain on surfaces with temperatures in this range (determined experimentally). 2. 2.Variation in the thermal and radiative environment is strongly correlated with variation in worker behaviour. To avoid stressful microhabitats, the workers abandon the surfaces of nests and unshaded foraging areas near midday on clear days. However, they do remain active on plants harbouring honeydew-secreting Homoptera and on shaded sections of foraging trails that constitute over 95% of the trail system between nests. Although bottlenecks do occur on the few unshaded portions of the trails, movement of workers transporting prey, nesting material, and honeydew along trails continues throughout the day.

Behavioural thermoregulation in three species of robber flies (Diptera, Asilidae: Efferia)

Abstract Robber flies of three species of the genus Efferia (Diptera: Asilidae) were active in a grassland habitat across a wide range of diurnal environmental temperatures. Perch height, posture and orientation of the body axes varied with environmental temperature. Early in the day, the flies warmed up by perching with their ventral surfaces close to the ground and basking with their longitudinal and dorsoventral body axes perpendicular to the direction of the sun. As the day progressed, they perched with their bodies raised off the surface and eventually moved onto surfaces above the ground, so that the air temperatures experienced remained within the relatively narrow range of approximately 27–35°C. Changes in perch height and orientation of the body axes resulted in significant changes in operative body temperatures. At the highest environmental temperatures, E. bicaudata and E. frewingi perched with their longitudinal axes towards the sun, while E. staminea showed no tendency to orient in any particular direction. The significance of behavioural thermoregulation and differences among the species in perch height and in orientation at high environmental temperatures are discussed.

Field‐scale Variations in Plant and Grasshopper Communities: A GIS‐based Assessment

GIS technology allowed us to examine species-specific occurrence and abundance patterns of important grassland herbivore species through the use of appropriate statistical methods and the superior spatial representation commonly available in existing GIS packages. The study was conducted on a 254 ha grassland area located 10 km south of Three Forks, Gallatin County, Montana, in the Agropyron spicatum province of the western steppe region of the U.S.A. From an ecological perspective, grasshopper species distribution and abundance patterns observed on a local, non-uniform landscape (2.6 km 2) were consistent with results found in other studies at the valley (≈1,400 km 2) and state-level (≈237,000 km 2) scales. Our observations at the local scale further demonstrate the importance of vegetation type and specific local stand physiognomies in structuring grasshopper populations. The application of results from studies conducted at various scales to the development of decision support tools for resource managers is also discussed.

Behavioural thermoregulation in two species of robber flies occupying different grassland microhabitats

1. 1.|The robber flies Machimus occidentalis and Machimus formosus are ambush predators that occur sympatrically in grassland habitats of southwestern Montana, U.S.A., although the former occurs earlier in the season. 2. 2.|Early in daily foraging periods, when ambient temperatures are relatively low, the two species exhibited similar behaviours. They perched in a crouched posture on fully insolated bare soil surfaces and oriented their longitudinal and dorsoventral axes to maximize the incident solar radiation intercepted by the broad lateral surface of the thorax. Measurements of operative body temperature of flies in different locations, orientations, and postures indicate that such basking can significantly raise body temperature. 3. 3.|Their thermoregulatory tactics diverged as daily temperatures and solar radiation loads rise. 4. 4.|Like previous grassland species studied, M. occidentalis moved upward onto vegetation, where crouched postures and orientation to the sun disappeared. 5. 5.|Machimus formosus, on the other hand, took refuge in the shaded portions of ground squirrel (Citellus sp.) and badger (Taxidea taxus) burrows. 6. 6.|In both cases, the vertical movements resulted in lower body temperatures than would occur if flies remained on the fully insolated soil surfaces. 7. 7.|We compare the tactics of these two species with those of previous grassland species studied.