Evaluating the Spatial Distribution of Dociostaurus Maroccanus Egg Pods Using Different Sampling Designs

Bulletin of Insectology 65 (2): 223-231, 2012 ISSN 1721-8861 Evaluating the spatial distribution of Dociostaurus maroccanus egg pods using different sampling designs 1 2 3 Ferdinando BALDACCHINO , Andrea SCIARRETTA , Rocco ADDANTE 1Department UTTRI-BIOTEC – ENEA C.R. Trisaia, Rotondella, Italy 2Department of Animal, Vegetal and Environmental Science - University of Molise, Campobasso, Italy 3Department of Agricultural and Environmental Biology and Chemistry - University of Bari, Italy Abstract In its gregarious phase the locust Dociostaurus maroccanus (Thunberg) (Orthoptera Acrididae) has periodically caused significant yield losses in many Mediterranean and Asian countries, and alarm in the general public. Population outbreaks in recent years have frequently required the application of control measures, based on those that have low environmental impact, which are only possible with a sound knowledge of locust bio-ethology and ecology. Our research was aimed at studying the spatial distribution of D. maroccanus egg pods in two Apulian egg bed areas in southern Italy, thus contributing to the rationalization of control methods. The distribution of D. maroccanus egg pods was investigated using a geostatistical approach. Three sampling designs (called A, B and C), characterized by different mesh and clod sizes, were compared to evaluate their effectiveness and affordability. In both egg bed areas, the variogram models were asymptotic with a small nugget effect, and indicated an aggregated distribution of egg pods. Contour maps showed that design A, based on a larger mesh and clod size, was characterized by few hot spots and an extended zone of “low density” egg pods, while design B, involving a smaller mesh and clod size, showed a more structured distribution, with various hot spots alternating with zero level zones. Finally, design C, based on a larger mesh size and smaller clods, showed a single extended hot spot surrounded by a large area without egg pods. Moreover, because of the larger amount of soil to be examined, design A was about 2.6- and 10.9-fold more time consuming than designs B and C, respectively. Our data showed that sampling designs providing smaller and denser samples should be preferred over designs with fewer and larger samples when information on both the distribution and density of egg pods is needed. Key words: Orthoptera, Acrididae, Apulia, geostatistic, oothecae distribution, bio-ethology. Introduction tification of these areas, the so-called egg beds, can be facilitated by following swarms of adult females to ovi- Dociostaurus maroccanus (Thunberg) (Orthoptera Ac- position sites. However, the availability of reports of rididae) is a univoltine species that is active from April breeding areas is not sufficient to trigger specific treat- (as nymphs) to August and spends the rest of the year as ment. The need for a spring treatment should take into eggs laid in oothecae, normally called egg pods, in the account: i) the extent of the egg beds (small isolated egg upper layers of the soil. beds are not necessarily a threat to adjacent crops); ii) In its gregarious phase, voracious swarms of this the density of vital eggs (taking into account that egg grasshopper have often occurred in regions of southern pods are subject to predation by some Diptera Bomby- Italy (Targioni Tozzetti, 1882; Melis, 1933; Jannone, liidae and Coleoptera Meloidae); iii) the trend in egg 1934; Paoli, 1937; Moleas et al., 2002), other Mediter- hatching (hatching is not uniform within a single egg ranean countries, Asia Minor, and the Middle East to bed, being influenced by soil temperature). Afghanistan and south-east Kazakhstan (Uvarov, 1932; Knowledge of the spatial distribution of egg pods is a 1933; Moreno Márquez, 1945; Delmas and Rambier, prerequisite for an efficient and economically sustain- 1951; Latchininsky and Launois-Luong, 1992; able strategy for assessing such factors. Visual monitor- Latchininsky et al., 2002), causing serious yield losses ing during the oviposition period can facilitate the loca- to cereals and other crops. tion of the egg bed areas, but only soil sampling can The environmental sustainability of a strategy aimed provide data on egg pod density, their rate of predation at controlling locusts is based on the use of active insec- and hatching. ticide substances with a low toxicity to non target or- The introduction of geostatistics in pest monitoring ganisms, on the timeliness of treatments and on restric- opened new possibilities for the study and management tion of treatment to the smallest possible area. Signifi- of the spatial patterns of insects at different stages. Geo- cant progress has been made in the search for locusti- statistics provides a powerful tool with which to model, cides with low environmental impact, spanning from estimate and represent spatially explicit correlated data organochlorine compounds to spinosyns, and on the use (Liebhold et al., 1993). Applications in applied ento- of entomopathogenic fungi such as Metarhizium ani- mology are various and include the study of the spatial sopliae (Metschnikoff) (Baldacchino et al., 2004). distribution and dispersion of populations at various Knowledge of the precise localization of D. maroc- scales, the identification of pest infestations in agro- canus hatching nymphs is required in order to perform ecosystems and the incorporation of spatial information timely treatment of the smallest possible area. The iden- into management plans (Brenner et al., 1998; Fleischer et al., 1999; Sciarretta et al., 2001; 2008; Nestel et al., tures were part of a wider garrigue phytoclimatic asso- 2004; Park et al., 2007; Albieri et al., 2010; De Luigi et ciation, characterized by an abundance of outcropping al., 2011; Burgio et al., 2012). rock and the presence of daffodils, thistles, squills, Geostatistical studies on Orthoptera are rather limited thyme and small trees or shrubs of wild pear, olive and and mainly focused on the adult stage. Johnson and terebinth. Fields surrounding the pastures were planted Worobec (1988) and Kemp et al. (1989) produced maps with wheat, barley, oats and forage crops, interspersed of grasshopper abundance on an area-wide basis; John- with olive and almond groves (Moleas et al., 2002). son (1989) analysed the spatial autocorrelation of grass- By following the ovipositing D. maroccanus females hoppers in roadside survey counts to predict population in June and July, egg beds were localized and appropri- densities in crop fields; locust spatial densities were cor- ately marked for sampling in subsequent seasons. related with host plants along the coastal plain in Sudan and distributional maps were obtained (Woldewahid et Field data collection and sampling procedure al., 2004). During the winter-spring period, samples of soil up to The aim of our study was to investigate the distribu- 10 cm in depth were taken from the egg beds in accor- tion of D. maroccanus egg pods in two egg bed areas in dance with the following sampling designs (figure 1): southern Italy, using different sampling designs. The - Sampling design A - Samples of soil of 50 × 50 cm in obtained spatial patterns were compared to assess their a grid with a mesh of 20 × 20 m; spatial resolution and the affordability of sampling de- - Sampling design B - Samples of soil of 15 × 15 cm in signs in terms of labour. a grid with a mesh of 10 × 10 m; - Sampling design C - Samples of soil of 15 × 15 cm in a grid with a mesh of 20 × 20 m. Materials and methods The third design (C) was a simulated sampling strategy, achieved by considering only the samples of soil in Study area design B which were in the same position as in A. In The study was carried out in pastures located near the this way, the resulting design was the same as that in A, town of Manfredonia in Apulia, Southern Italy. For the but with sample sizes of 15 × 15 cm. purposes of this study only data obtained from egg beds General information on the designs, including the di- in pastures close to the localities Posta del Fosso mensions of the sampled clods and the distances be- (41°38'25.17"N, 15°52'51.43"E, at 70 m a.s.l.) and tween sampling points, is given in table 1. Posta Garzia (41°38'6.73"N, 15°52'55.61"E, at 57 m Samples of 50 × 50 cm were transported to the labora- a.s.l.) were used in the geostatistical analysis, as those tory and examined for egg pods, while the clods of 15 × sites were the largest and richest in egg pods. The pas- 15 cm were examined directly in the field. Figure 1. Spatial pattern of sampling points according to different experimental designs (A, B and C) at Posta del Fosso and Posta Garzia. 224 Table 1. General characteristics of the sampling designs with clods of 50 × 50 cm (design A) and 15 × 15 cm (design B and C) at the sites Posta del Fosso and Posta Garzia. Posta del Fosso Posta Garzia Plan A Plan B Plan C Plan A Plan B Plan C Surface area of laying sites (ha) 2.9 3 2.9 2.9 3.2 2.9 Number of samples 80 340 80 85 360 85 Distance between samples (m) 20 × 20 10 × 10 20 × 20 20 × 20 10 × 10 20 × 20 Maximum distance (m) 240 250 240 240 255 240 Surface area of single sample (m2) 0.25 0.02 0.02 0.25 0.02 0.02 Total surface area sampled (m2) 20 7.65 1.8 21.25 8.1 1.9 Total volume sampled (m3) 2 0.76 0.18 2.12 0.81 0.19 Surface area sampled (%) 0.070 0.025 0.006 0.070 0.025 0.006 Given the different grid and clod sizes used in the ex- tested by the software.

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