POPULATION ECOLOGY Spatial Analysis of Entomopathogenic Nematodes and Insect Hosts in a Citrus Grove in a Semi-Arid Region in Israel DAVID EFRON, 1 DAVID NESTEL, 2 AND ITAMAR GLAZER 1, 3 Environ. Entomol. 30(2): 254Ð261 (2001) ABSTRACT We investigated the spatial and temporal dispersion patterns of entomopathogenic nematodes (Steinernematidae and Heterorhabditidae) and their host insects in a citrus orchard located in the semiarid region of Israel (the Negev). Spatial patterns of entomopathogenic nema- todes were studied in plots differing on their shade level, and at two geographic scales: a macro-scale, which included the entire orchard, and a micro-scale, which investigated the spatial patterns around individual trees. Spatial patterns of insect hosts were only studied at the macro-scale. At the macro-scale, soil samples were obtained from a symmetrical grid in which samples were separated by a distance of 20 m; and in the micro-grid, soil samples were separated by a distance of 50 cm. Population levels of entomopathogenic nematodes were spatially autocorrelated using MoranÕs I statistics. Population loads of entomopathogenic nematodes, and their spatial and temporal patterns, were not affected by the orchardÕs shade level. Entomopathogenic nematode loads were higher during the fall, which coincides with the reported abundance of their main hosts in this region, grubs of Maladera matrida (Argaman). At the investigated scales and distances, we were unable to detect any spatial dependence of entomopathogenic nematode populations or of insect hosts. However, clusters of entomopathogenic nematodes were found to be highly localized in small volumes of soil. At the level of the entire plot area we found a very strong correlation between the number of host insects at a particular time and the level of entomopathogenic nematodes (as indicated by a laboratory bioassay) 1 mo after (shifted-back correlation). We conclude that entomopathogenic nematodes in the Negev orchards have a very low probability of encountering insect hosts, and that under the environmental conditions of the Negev, effective biological control with entomopatho- genic nematodes can only be achieved by inundative releases. KEY WORDS population dynamics, entomopathogenic nematode, spatial pattern ENTOMOPATHOGENIC NEMATODES OF the families Stein- ernematidae and Heterorhabditidae are known as ef- fective biological agents against soil dwelling stages of insect pests (Kaya and Gaugler 1993, Martin 1997). These insect pathogens are mutualistically associated with particular bacteria (Xenorhabdus spp. for Stein- ernematidae and Photorhabdus spp. for Heterorhab- ditidae, Frost et al. 1997). The efÞcacy of the nema- tode-bacterium complex against insects and the environmental conditions affecting their activity have been reviewed previously (Glazer 1992, 1996; Kaya and Gaugler 1993). Furthermore, numerous studies have demonstrated their ability to reduce insect pop- ulations when inundatively applied (Georgis and Manweiler 1994, Martin 1997). Entomopathogenic steinernematids and hetero- rhabditids have been isolated from diverse natural habitats around the globe (Hominick et al. 1996). Although a vast amount of information exists on en- tomopathogenic nematode infectivity of insects in lab- oratory and Þeld conditions, little is known about the ecology of these organisms and their interaction with their insect hosts in nature. Most of the environmental studies conducted on this group consist of nematode surveys (Akhurst and Brooks 1984; Akhurst and Bed- ding 1986; Blackshaw 1988; Hominick and Briscoe 1990a, 1990b; Glazer et al. 1991; GrifÞn et al. 1991; Goede and de Bongers 1994). Few studies have actu- ally attempted to investigate the spatial and temporal distribution patterns of entomopathogenic nematodes in their natural habitats (Brodie 1976, Campbell et al. 1996, Glazer et al. 1996, del Pino and Palalomo 1997) or the interactions of entomopathogenic nematodes with their insect hosts (Strong et al. 1995, 1996). In a survey that we conducted in the Negev, the arid region of Israel, entomopathogenic nematode popu- lations from the family Heterorhabditidae were iso- lated from orchards in the northwestern part of this region (Glazer et al. 1991). This area has sandy soil and nematodes have never been isolated from nonculti- vated soils. A further study in this area included the characterization of the temporal occurrence of ento- mopathogenic nematodes in an orange grove, and the interrelationship between the presence of nematodes and some environmental parameters such as soil depth, distance from tree trunk, and shade (Glazer et al. 1996). Our previous results suggested a strong re- lationship between nematode densities and shade in- tensity (Glazer et al. 1996). However, because of the 1 Department of Nematology, Volcani Center, Bet Dagan, 50250, Israel. 2 Department of Entomology ARO, Volcani Center, Bet Dagan, 50250, Israel. 3 To whom reprint requests should be addressed. 0046-225X/01/0254Ð0261$02.00/0  2001 Entomological Society of America Downloaded from https://academic.oup.com/ee/article/30/2/254/448218 by guest on 29 June 2022