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- Materials and methods
1. Rangeland grasshopper movement was studied in Wyoming, USA, with respect to the biological and ecological factors (population density, developmental stage and weather) influencing net displacement and directionality.
2. A novel adaptation of the mark–recapture method was developed to monitor grasshopper dispersal. The method used fluorescent powder and resighting marked grasshoppers in the field with ultraviolet light, rather than physical recapturing of individuals.
3. Rangeland grasshoppers exhibited a strong tendency for directional movement. Adult grasshoppers demonstrated a significant tendency for dispersal in a north-westerly direction across a range of population densities (5–8, 10–15 and ≥ 18 grasshoppers m–2). Although not a definitive explanation, weather might have influenced this behaviour, as the grasshoppers consistently moved upwind.
4. The mean displacement of grasshoppers in a 36-h period ranged from 2·3 m in nymphs to 3·7 m in adults, with the distance of displacement being positively correlated with population density.
5. An understanding of grasshopper movement in terms of directionality and displacement has immediate applicability to reduced agent–area treatments for rangeland grasshopper management.
- Top of page
- Materials and methods
The rangeland grasshopper control method historically and currently used in North America is a ‘blanket’ treatment with broad-spectrum insecticides. However, this traditional method is costly, and available evidence shows that such large-scale insecticide applications have serious effects on beneficial and non-target organisms (Tepedino 1979; USDA 1987; Lockwood, Kemp & Onsager 1988). Unfortunately, other management methods (e.g. biological control and rotational grazing) are not mature enough in terms of application methods, product development and efficacy to be widely used in rangeland grasshopper programmes.
Although qualitatively different strategies are not available, quantitative modifications of chemical control have been sufficiently tested and refined to have immediate applicability (National Grasshopper Management Board 1998). One of the new, alternative control methods involves treating only a portion of the infested area with very low rates of insecticides in reduced agent–area treatments (RAAT; Lockwood & Schell 1997) instead of the traditional or standard blanketing of the whole infestation area at relatively high rates. In large part, this method apparently depends on grasshoppers in untreated areas moving into the treated areas and subsequently ingesting the insecticides. As such, the insecticide applied to the treated swaths actually works on both the grasshoppers in the immediate area and those that immigrate from the adjacent, untreated swaths. Thus, grasshopper movement is one of the key elements determining the efficacy of the RAAT method (Lockwood & Schell 1997), and this phenomenon is the focus of the present project.
A number of authors have studied movement of one or a few particular grasshopper species in experiments across a range of conditions (Riegert, Fuller & Putnam 1954; Clark 1962; Aikman & Hewitt 1972; Mason, Nichols & Hewitt 1995). These studies have indicated that the net displacement of grasshoppers varies as a function of species and development. The movement of grasshoppers was influenced by the microhabitat conditions, with these insects sometimes exhibiting site attachment, which made the movement more complex. Interpretation of these studies is difficult due to a number of methodological limitations. Normal movement was confounded by the effects of initial, agitation-based dispersal, overcrowding dispersal, and human disturbances during release and recapture of the grasshoppers. Furthermore, these works concentrated on only a few individual grasshopper species, some of them flightless (e.g. Podisma pedestris L.; Mason, Nichols & Hewitt 1995). In some cases, the sites of the studies were abnormally simplified, including a golf course (Aikman & Hewitt 1972) and bare ground (Riegert, Fuller & Putnam 1954), so extrapolation to the structural complexity of rangeland is tenuous. Finally, previous studies have not addressed the effect of density on grasshopper movement, and only one study (Riegert, Fuller & Putnam 1954) analysed dispersal as a function of development. Although these studies provide insight into specific aspects of grasshopper behaviour, the results are difficult to apply in a pest management context, where grasshoppers occur at high densities in complex rangeland habitats. Therefore, our goal was to investigate the potential effects of density and development on the distance and directionality of grasshopper movement in naturally occurring species’ complexes found in native, rangeland habitats.
Considerable success has been obtained using fluorescent dye to mark, and ultraviolet (UV) light to detect recaptured (marked) individual insects. The method is also advantageous because the insects do not have to be individually handled, marked or recovered. The first such studies were conducted with mosquitoes (Zukel 1945; Chang 1946; Pal 1947; Reeves, Brookman & Hammon 1948), tsetse flies (Jewell 1958) and aphids (Post & Anderson 1950). More recently, this approach has been employed in movement studies of parasitoid wasps (Corbett & Rosenheim 1996) and a wide range of Coleopteran pests of crops and forests (Gara 1967; Schmitz 1980; Linton et al. 1987; McMullen et al. 1988; Shore & McLean 1988; Salom & McLean 1989; Naranjo 1990; Oloumi-Sadeghi & Levine 1990). These studies have found little effect of marking on the behaviour of the insects. Most recently, Cook & Hain (1992) concluded that the marking had no effect on the flight initiation and semiochemical perception of bark beetles, and the marking remained intact on stored, dried beetles. Although the marking decreased the adult life span in this particular study, it was not of concern in the context of short-term monitoring. Thus, with this method, it is possible to monitor dispersal of highly mobile insects without affecting their normal behaviour.
Fluorescent dyes have not been used extensively to study grasshopper movement. This approach was considered for monitoring the dispersal of insecticides in locust control programmes (G. Bruge, personal communication). A single grasshopper (Melanoplus sp.), accidentally marked with fluorescent dye during a study of sawfly migration was recovered (Post & Anderson 1950), but intentionally marking grasshoppers with fluorescent dye previously has not been attempted. As such, we developed and implemented this method as a tool for tracking the movement of rangeland grasshoppers.