DOI: 10.1111/j.1570-7458.2006.00498.x Blackwell Publishing Ltd Regional abundance of a planthopper pest: the effect of host patch area and configuration Mariano P. Grilli* & Marina Bruno CONICET – Centro de Relevamiento y Evaluación de Recursos Agrícolas y Naturales, Facultad de Ciencias Agropecuarias, Universidad Nacional de Córdoba, Av. Valparaiso s/n, CC 509, 5000, Córdoba, Argentina Accepted: 26 September 2006 Key words: Delphacid planthopper, Delphacodes kuscheli, disease vector, Landsat 5 TM, Homoptera, Delphacidae Abstract The effect of host patch area and configuration on the abundance of dispersing individuals of Delpha- codes kuscheli Fennah (Homoptera: Delphacidae), the vector of Río Cuarto disease in maize, was investigated in the main maize production area of Argentina. Actively dispersing D. kuscheli indivi- duals were collected from 15 sampling sites during the spring seasons of 1999 and 2000, using sticky traps placed at 6 m above ground level. Host patches were detected and quantified using Landsat 5 TM images for the periods studied. The spatial pattern analysis program FRAGSTATS was used to estimate the total class area, largest patch index, mean proximity index, and patch cohesion index for patches of winter pastures (the main insect host during winter) as observed from the satellite images. Landsat 5 TM estimations showed local variability in the proportion of winter pastures, with patches bigger during 1999 than during 2000, but these patches represented only a very small part of the total landscape. Proximity between host patches was also variable between sites and higher values of cohe- sion occurred during the first sampling season. The relationship between host area and D. kuscheli mean abundance was adjusted to an exponential (R2 = 77.5%) model. Host patch dominance, host patch isolation, and host patch connectivity all showed a positive relationship with D. kuscheli mean abundance, adjusting significantly to linear models (R2 = 92%, R2 = 90%, and R2 = 22%, respectively). Outbreaks of Río Cuarto disease in the main maize production area of Argentina are related to high vector populations. The results indicate that the abundance of D. kuscheli depends on factors related to the abundance and configuration of its host patches. wheat (Triticum aestivum L.) and winter pastures such as Introduction oats (Avena sativa L.). The latter are the most important Delphacodes kuscheli (Fennah) (Homoptera: Delphacidae) overwintering hosts as they are sown by the end of the is the economically most important delphacid species in summer and not harvested until spring, becoming the Argentina because of its ability to transmit what was initially main source from which D. kuscheli migrates to maize thought to be a local strain of the maize rough dwarf virus fields (Tesón et al., 1986; Virla & Remes Lenicov, 1991; (MRDV–RC) named Río Cuarto disease (Conci & Marzachi, Ornaghi et al., 1993; Garat et al., 1999; Remes Lenicov 1993; March et al., 1995). The insect does not reproduce et al., 1999). The species has a wide distribution, from the on maize, but it can transmit the virus when feeding on north of Argentina (Jujuy Province) to the south (Río maize plants. If the infection occurs during the first 3 weeks Negro Province) (Remes Lenicov et al., 1999). after plant emergence, the disease can be severe and, in There are two wing forms among populations of some cases, lead to plant death. After its most susceptible D. kuscheli: long-winged macropters, which can fly, and period, infection may occur but the effect on grain short-winged brachipters, which are flightless (Ornaghi production is relatively minor (Lenardón, 1987). et al., 1993). Only macropters disperse and, in general, Delphacodes kuscheli has a limited range of hosts, and their dispersal range is from 1 to 3 km (Denno & Grissel, can breed on winter cereals such as rye (Secale cereale L.) or 1979; Denno et al., 1980, 1991). Delphacodes kuscheli is a multivoltine species, with three or four generations per *Correspondence: E-mail: [email protected] year (Remes Lenicov et al., 1991). Most of the generations © 2006 The Authors Entomologia Experimentalis et Applicata 122: 133–143, 2007 Journal compilation © 2006 The Netherlands Entomological Society 133 134 Grilli & Bruno appear during late (southern hemisphere) spring and early south of the provinces of Córdoba and Santa Fé to the summer (Garat et al., 1999), with a clear seasonal pattern, north of the province of Buenos Aires. There are 16 species increasing during October and November, which roughly of Delphacidae present in the study area, but D. kuscheli is coincides with the senescence of the winter pastures, and the most abundant, with the highest ‘relative weight index’ peaking in December (Grilli & Gorla, 1999, 2002). Areas (approximately 25) (Laguna et al., 2002). In other areas, with high population densities are strongly associated with insect abundance and disease incidence are variable. land use management (Grilli & Gorla, 1997, 1998), and the presence, condition, and distribution of host vegetation Insect sampling (Grilli & Gorla, 1997, 1998). The abundance of planthop- Data on insect abundance were derived from an 8-month pers is related to the distribution and abundance of host field study that recorded flying D. kuscheli individuals using plants: the higher the environmental diversity in terms of sticky traps. Insects were collected within a 59 113 km2 area crop species per unit area, the lower the abundance of using sticky traps, as described in Grilli & Gorla (1997), individual planthopper species (Grilli & Gorla, 1999). But replaced every 7 days during the spring and summer all these studies were performed at a regional level. seasons of 1999 and 2000. This method has been shown In the study area, farmers are very traditional in terms of to be useful for estimating the abundance of dispersing when and what to sow in their farmland, and crop rotation macropterous individuals (Grilli & Gorla, 1998). Sampling in each field on the farm is a common practice. In general, was conducted from the beginning of spring in order to it is thus very rare for a farm plot to have the same crop detect flying D. kuscheli individuals leaving winter pastures, from one year to the next. The same place will show a which are the only host available from the end of summer changing pattern of land use, with larger or smaller areas to the end of winter. Sampling was carried out at 15 sites, dedicated to host and non-host-plant species, and this can eight in 1999 (identified as A, B, C, D, E, F, G, and H) and be considered as the most important change observed in seven in 2000 (identified as I, J, K, L, M, N, and O; Figure 1). the land. Sites were 50 km apart and the sticky traps in each of them Many authors (Chen et al., 1995; Hunter et al., 1996, consisted of metal cylinders, supporting a plastic film coated 2002; Collinge, 2000; Connor et al., 2000; Hanski & Singer, with lithium grease (YPF® EP 62, Repsol-YPF, Argentina) 2001; Biederman, 2002; Cronin, 2003) have emphasised as an adhesive, which were placed on the top of a 6-m mast. the role of habitat patches in insect population ecology. The plastic film was removed on each sampling date, Studies of patchily distributed populations showed the replaced by a clean one and taken to the laboratory, where importance of patch size and isolation in determining their D. kuscheli were identified according to Remes Lenicov & distributions (Hanski, 1999). The occurrence and density Virla (1999). At each sampling site, three traps were placed of planthoppers in a particular habitat patch may depend to form a single set (maximum of 100 m and minimum of on the area, isolation, quality, and surrounding landscape 25 m separation between the most distant traps of the set) structure of the patch (Biederman, 2002). In many cases with no special connection between them. Sites were immigration and emigration from habitat patches will be selected within a uniform landscape, and special care was affected by patch size and distribution (Connor et al., taken to avoid tree barriers in the area around the traps. 2000; Cronin, 2003). Insect abundance was expressed as insects/trap/day. The size, number, and distribution of patches are important Although the traps were in the field from September to aspects of a landscape (Forman & Godron, 1986). The December (the most critical period for Río Cuarto disease spatial configuration of a landscape can be quantified using transmission), the mean insect abundance estimated patch-based measures (Gustafson, 1998), including size, considered only the spatial variation of the abundance number, density, and connectivity of patches, computed for with all the insects captured during this period (121 days) all classes at the same time or for a particular class of interest grouped in one mean value per site. (Gustafson & Parker, 1992; Schumaker, 1996; Gustafson, 1998). The objective of this work was to examine the effects Land-cover estimation of host patch area and configuration on the abundance of Winter pastures are sown at the end of the summer and are the dispersing fraction of D. kuscheli populations. not harvested until spring. Eight Landsat TM 5 scenes were used to estimate the land cover of the study area during each year of the study. A supervised classification was used Materials and methods to estimate land use, based on spectral brightness, for six Study area spectral bands in the visible and reflected infrared regions The study was performed in the most important maize of the electromagnetic spectrum. We identified 300 training production area of Argentina, which extends from the sites from site visits and considered three classes in the Planthopper abundance and host patch configuration 135 Figure 1 Areas around the sampling site with host patches extracted from Landsat 5 TM classified images.