Stable Fly, Stomoxys calcitrans (L.), Dispersal and Governing Factors

Although the movement of stable fly, Stomoxys calcitrans (L.), has been studied, its extent and significance has been uncertain. On a local scale (<13 km), fly movement occurs between host animals and resting sites to feed and mate, mainly at on-farm locations where herbivorous livestock regularly congregate. Small numbers emigrate from livestock congregation sites in search of other hosts and oviposition substrate, mostly within <1.6 km. Such local movement occurs by flight ~90 cm above ground, or with moving livestock. While stable flies are active year-round in warm latitudes, cold winters in temperate areas result in substantial population and activity declines, limiting movement of any sort to warmer seasons. Long-distance dispersal (>13 km) is mainly wind-driven by weather fronts that carry stable flies from inland farm areas for up to 225 km to beaches of northwestern Florida and Lake Superior. Stable flies can reproduce for a short time each year in washed-up sea grass, but the beaches are not conducive to establishment. Such movement is passive and does not appear to be advantageous to stable fly's survival. On a regional scale, stable flies exhibit little genetic differentiation, and on the global scale, while there might be more than one "lineage", the species is nevertheless considered to be panmictic. Population expansion across much of the globe likely occurred from the late Pleistocene to the early Holocene in association with the spread of domesticated nomad livestock and particularly with more sedentary, penned livestock.

Forecasting spatially structured populations: the role of dispersal and scale

We forecasted spatially structured population models with complex dynamics, focusing on the effect of dispersal and spatial scale on the predictive capability of nonlinear forecasting (NLF). Dispersal influences NLF ability by its influence on population dynamics. For simple 2-cell models, when dispersal is small, our ability to predict abundance in subpopulations decreased and then increased with increasing dispersal. Spatial heterogeneity, dispersal manner, and environmental noise did not qualitatively change this result. But results are not clear for complex spatial configurations because of complicated dispersal interactions across subpopulations. Populations undergoing periodic fluctuations could be forecasted perfectly for all deterministic cases that we studied, but less reliably when environmental noise was incorporated. More importantly, for all models that we have examined, NLF was much worse at larger spatial scales as a consequence of the asynchronous dynamics of subpopulations when the dispersal rate was below some critical value. The only difference among models was the critical value of dispersal rate, which varied with growth rate, carrying capacity, mode of dispersal, and spatial configuration. These results were robust even when environmental noise was incorporated. Intermittency, common in the dynamics of spatially structured populations, lowered the predictive capability of NLF. Forecasting population behaviour is of obvious value in resource exploitation and conservation. We suggest that forecasting at local scales holds promise, whereas forecasting abundance at regional scales may yield poor results. Improved understanding of dispersal can enhance the management and conservation of natural resources, and may help us to understand resource-exploitation strategies employed by local indigenous humans.

Biological control of locusts and grasshoppers

Control of grasshoppers and locusts has traditionally relied on synthetic insecticides, and for emergency situations this is unlikely to change. However, a growing awareness of the environmental issues associated with acridid control as well as the high costs of emergency control are expanding the demand for biological control. In particular, preventive, integrated control strategies with early interventions will reduce the financial and environmental costs associated with large-scale plague treatments. The recent development of effective oil formulations of Metarhizium anisopliae spores in Africa, Australia, and Brazil opens new possibilities for environmentally safe control operations. Metarhizium biopesticide kills 70%-90% of treated locusts within 14-20 days, with no measurable impact on nontarget organisms. An integrated pest management strategy, with an emphasis on the use of Metarhizium, that incorporates rational use of chemical pesticides with biological options such as the microsporidian Nosema locustae and the hymenopteran egg parasitoids Scelio spp., has become a realistic option.

Are recession populations of the desert locust (Schistocerca gregaria) remnants of past swarms?

The desert locust (Schistocerca gregaria) undergoes crowding-induced phase transformation from solitary to gregarious, which involves changes in behaviour, colour, development, morphometry, fecundity and endocrine physiology. During recession, solitary locusts persist in the central, drier part of the species' range in small pocket populations that are prone to extinction. During the intermittent upsurges and the subsequent plagues, gregarious swarms attain huge population size and invade a vast area causing major damage to agriculture. A highly variable nuclear DNA marker, a noncoding 3' end fragment of an antennapedia-class homeobox gene, was screened in locust samples from Eritrea. Despite the homogenizing potential of plague swarms, the last of which was in 1986-89 and originated in this region, the population genetic structure of solitary phase locusts along the Red Sea coast of Eritrea revealed significant divergence. The pattern of divergence indicated that the invasion of the western and northern plains in the summer of 1995 may not, as reported then, have originated in eastern Chad or western Sudan. A number of interrelated hypotheses have been presented to explain the observed genetic heterogeneity between the sampled populations. We conclude, with caution due to the limited sample sizes, that: (i) geographical isolation between breeding sites during plagues and recession; (ii) the marked differences in the flight behaviour of plague swarms and recession populations; (iii) possible failure of gregarious locusts to solitarize and re-establish in recession areas; and (iv) the effect of repeated extinction and recolonization in the meta-population contribute to the maintenance of the genetic structure of recession populations. Potentially productive future research has been identified.

Expression of phase-specific haemolymph polypeptides in a laboratory strain and field catches of Schistocerca gregaria

Uvarov's theory of locust phase polymorphism implies a differential phase-specific gene expression. It was the goal of the present investigation to provide data on the molecular level for this concept. We used the technique of 2D gel electrophoresis to generate haemolymph polypeptide maps from mature locust males. Under the given conditions we identified 238 polypeptide spots by their molecular weight and isoelectrical point. Isolated and crowded males from the Mainz strain, all originating from crowded ancesters, differed in 20 spots: three were solitary-specific and 17 crowded-specific. Field catches of solitary and gregarious S. gregaria in Mauritania revealed that those males showed the phase-specific expression of the same 20 polypeptide spots as the Mainz strain. (It should be mentioned that the field catches and the Mainz animals differed in nine spots, but none of them was phase-specific.) As a crosscheck we reared originally solitary field catches for two generations under crowded conditions. This resulted in the repression of one solitary-specific spot and the expression of 14 gregarious/crowded-specific spots. Since JH conceivably plays a decisive role in the manifestation of the solitary phase we applied the JH analogue fenoxycarb to crowded Mainz males. 15 days after the treatment nine of the 17 gregarious-specific polypeptide spots were repressed. The fact that experimental manipulations-limited by time-cannot alter the expression of all phase-specific polypeptides may reflect the observation that, in nature, some phase characteristics can change within hours whereas others need several generations. The results provide evidence that a number of haemolymph polypeptides are expressed and repressed, respectively, in relation to phase.

The physiology of locust phase polymorphism: an update

The considerable progress made between 1990 and 1997 in locust phase-related research and in understanding the physiology of locust phase polymorphism is reviewed. The traits of locust phases are discussed and it is concluded that there are distinct strain-dependent differences in phase characteristics and their amplitudes even in the same species. Despite some advances, no major break-through was achieved in the putative endocrine control of locust phase polymorphism. Phase-dependent differences in adipokinesis, flight fuels and migration of adult locusts, as well as novel methods in studying aggregation behaviour and activity of hoppers and adults, opened new lines in research of the physiology of locust phase polymorphism. Marked advances were made in phase-related locust pheromone research, revealing, in Schistocerca gregaria, differences between the pheromonal system of the hoppers and that of the adults. These systems turned out to be more complex than previously assumed. Phenylacetonitrile, produced by sexually mature adult males, serving both as an attractant and a mutration-accelerating factor, was identified as the major compound of the adult pheromonal system in S. gregaria. A new aspect of transmission of phase characteristics from parent to progeny through the foam (froth) of the egg pod was revealed. Effects of some plant substances on locust phases were reported. However, no research has yet been published on the aspects of molecular biology of locust phase polymorphism.