Eur. J. Entomol. 109: 561–568, 2012 http://www.eje.cz/scripts/viewabstract.php?abstract=1742 ISSN 1210-5759 (print), 1802-8829 (online) Dispersal of Aphytis melinus (Hymenoptera: Aphelinidae) after augmentative releases in citrus orchards LUCIA ZAPPALÀ1, ORLANDO CAMPOLO2, SAVERIO BRUNO GRANDE2, FRANCESCO SARACENO3, ANTONIO BIONDI1, GAETANO SISCARO1 and VINCENZO PALMERI2 1 Department of Agri-food and Environmental Systems Management, University of Catania, via Santa Sofia 100, 95123 Catania, Italy; e-mails: [email protected]; [email protected]; [email protected] 2 Department of Agricultural and Forest Systems Management, University of Reggio Calabria, loc. Feo di Vito, 89122 Reggio Calabria, Italy; e-mails: [email protected]; [email protected]; [email protected] 3Sicilian Regional Food and Agriculture Assessorship, Department of Structural Intervention in Agriculture, Plant Protection Serv ice U.O. 42, Via Sclafani 34, 95024 Acireale (CT), Italy; e-mail: [email protected] Key words. Aphelinidae, California red scale, biological control, parasitoid, flight range, spatial interpolation Abstract. The efficacy of natural enemies in controlling pests under field conditions is largely correlated with their capacity to spread within infested crops. In this study the spatial dispersal of the California red scale parasitoid Aphytis melinus DeBach (Hymenoptera: Aphelinidae) was evaluated in the field after augmentative releases. The experiment was conducted in 2007 in six 1-ha plots in a Sicilian citrus orchard under integrated pest management. A total of 180,000 A. melinus adults was released in each of three plots and the other plots were left as untreated control. The flight range of the parasitoid was evaluated, for 35 days after the release, on 16 trees per each plot, located at 20 and 40 m from the central release point using yellow sticky traps activated with Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae) sexual pheromone and by monitoring the percentage parasitism of the scale on fruits and twigs. The effects of the distance from the release point and density of susceptible stages of host on parasitoid dispersal were evaluated. The number of wasps captured during the whole trial was greater in the traps located 20 m from the release point than in those at 40 m and in the control plots. Aphytis melinus dispersed over distances less than 40 m based on both the lower per- centage parasitism and numbers captured recorded at distances of 40 m. The results are discussed in the context of the biological control of California red scale in citrus orchards by means of wasp releases. In particular, the release points should be no more than 40 m apart for a quick and homogeneous colonization of the area treated. INTRODUCTION In addition, one of the attributes of effective natural Knowledge of the mobility and dispersal capacity of enemies is their ability to aggregate spatially in response entomophagous arthropods is essential for the implemen- to the patchy distributions of their hosts, which results in tation of biological control strategies. In augmentative female parasitoids spending more time and/or concen- programs, when the aim is a quick effect of a mass release trating in areas where hosts are more abundant (Sanchez of natural enemies rather than their establishment, benefi- et al., 2009). Thus many authors assumed that an aggre- cials must move from the release points and spread gative response would increase the parasitoids foraging throughout the infested area (Corbett & Rosenheim, efficiency and lead to direct density-dependent parasitism 1996). However, a high tendency to disperse could lead and regulation of the host population (Hassell & May, to ineffective control (Hougardy & Mills, 2006). In addi- 1974; Heads & Lawton, 1983; Stiling, 1987). However, tion to having a knowledge of the dispersal abilities of in several cases it was shown that spatial aggregation of biocontrol agents it is also important to know how effec- parasitism in response to local host density is not a neces- tive they are at recolonizing crops from untreated sur- sary condition for stability or for successful biological roundings after planting or harvesting and especially after control (Reeve & Murdoch, 1985) and that inverse den- pesticide applications (Desneux et al., 2005, 2007; sity dependent and density-independent patterns of para- Langhof et al., 2005). Despite the importance of the dis- sitism may also be potential stabilizing mechanisms if the tribution of natural enemies for biological control, there distribution of parasitism is sufficiently aggregated (Mor- are few studies on the dispersal behaviour of biocontrol rison & Strong, 1980; Murdoch et al., 1985; Stiling, agents, mainly because it is difficult to measure the dis- 1987; van Veen et al., 2002). Therefore, the dispersal and persal ability of small insects (Hougardy & Mills, 2006; host-location behaviour of wasps, especially released Tabone et al., 2010). The techniques used up to now rely ones, are factors that clearly influence the efficiency of on recording parasitism percentage on natural or artificial biocontrol (Suverkropp et al., 2009; Tabone et al., 2012). hosts, capture numbers on sticky traps or on glue-sprayed The patterns of distribution vary between species and are plants and/or by using mark-recapture (Keller et al., 1985; also influenced by release conditions and physical factors Corbett & Rosenheim, 1996; Suverkropp et al., 2009; Yu such as wind, temperature and vegetation structure. Wind et al., 2009). 561 in particular seems to be an important factor for minute parasitic Hymenoptera (Corbett & Rosenheim, 1996). In this study the ability of the parasitic wasp Aphytis melinus DeBach (Hymenoptera: Aphelinidae) to disperse in a citrus orchard was investigated. This parasitoid is important in controlling California red scale, Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae), (Sorribas & Garcia-Marí, 2010) also by means of augmentative releases (Moreno & Luck, 1992). This scale is considered one of the most important pests of citrus in the Mediterra- nean basin as well as in other citrus growing areas world- wide (Jacas et al., 2010). It attacks all aerial parts of the tree including twigs, leaves, branches and fruit. Heavily infested fruit may be downgraded in the packinghouse and, if population levels are high, trees can be seriously damaged. Aphytis melinus effectiveness could depend on the scale careful monitoring, on the use of selective insec- ticides to control other pests (Grafton-Cardwell et al., 2006; Suma et al., 2009; Planes et al., 2012; Vanaclocha et al., 2012), on the host instars available and their size (Luck & Podoler, 1985; Pekas et al., 2010a), on ant activity (James et al., 1997; Pekas et al., 2010b), on the fitness of the released insects (Vasquez & Morse, 2012) or on environmental conditions (DeBach & Sisojevic, 1960). The aim of this work was to evaluate the dispersal ability of released A. melinus adults and the spatial pat- tern of parasitism, using A. aurantii pheromone yellow sticky traps and scoring percentage parasitism on infested twigs and fruit. Using these methods has the advantage of providing both qualitative and quantitative data on the wasp’s presence and distribution in space. The density of Fig. 1. Outline of the positions of the experimental fields in the host, the spatial distribution of adult and pre-imma- the surrounding landscape. The location of the trees sampled around the central release point is shown in box (A). ginal stages of both host and parasitoid and the per- centage of parasitism are reported. The ability of A. melinus to disperse in the field was poorly studied in the Insect releases past (Samways, 1986) or only over a period of several The wasps used in the experiment were reared at the Sicilian generations (Campbell, 1976). A good knowledge of its Regional Insectary on a parthenogenetic strain of oleander scale, dispersal ability is particularly relevant considering the Aspidiotus nerii Bouché (Hemiptera: Diaspididae), fed on squash (Cucurbita maxima Duch. var. Butternut) (Raciti et al., extremely patchy distribution of the host species (Meats 2003). In order to keep background parasitism by naturally & Wheeler, 2010) as well as A. melinus poor ability to occurring A. melinus low, the trial was carried out in a period disperse and short adult life (Rosen & DeBach, 1979; (June–July) when the parasitoid is scarce in the field (Lizzio et Samways, 1986; Heimpel et al., 1997). The results al., 1998; Siscaro et al., 1999) and California red scale is mainly obtained are discussed in the context of biological control present as virgin females, which is the preferred instar of the with specific reference to the ability of A. melinus to dis- parasitoid (Luck & Podoler, 1985; Heimpel et al., 1997; Pekas perse after augmentative releases in citrus orchards. et al., 2010a). In each of the three treated plots, 180,000 A. melinus adults MATERIAL AND METHODS were released on the central tree on a single date, while the other plots were used as untreated controls (Fig. 1). This number Experimental field of A. melinus adults is usually released annually per hectare The trial was carried out in June–July 2007 in a citrus orchard spread over several dates in spring, early summer and autumn under integrated pest management located at Lentini (province (Moreno & Luck, 1992; Zappalà et al., 2008). The adult parasi- of Siracusa, Italy) (37°20´34˝N; 14°49´44˝E) at 80 m above sea toids released were less than 48 h old, collected by anesthetizing level, where no chemical treatments had been applied for three them using CO2 , and the number quantified based on a volume years prior to the trial. The trees in the experimental field were estimation (DeBach & White, 1960; Raciti et al., 2003). They 20 year-old blood orange trees (cv Tarocco, clone Sciré) planted were then divided up and groups of 5,000 adults were placed in in a 5 by 5 m grid. The trial was conducted in six 1-ha plots, 150 ml vials.