Biological Control of Hazelnut Weevil (Curculio nucum L., Coleoptera, Curculionidae) Using the Entomopathogenic Fungus Beauveria bassiana (Balsamo) Vuill. (Deuteromycotina, Hyphomycetes)

The need to biologically control phytophagous insects has led researchers to consider using microorganisms to control insect pests in the field. On this basis, we decided to conduct field tests to determine the effectiveness of a commercial product made of Beauveria bassiana (Bals.) against the key hazelnut pest Curculio nucum L. Trials were carried out in a hazelnut orchard in the province of Viterbo (Italy). Three untreated control cages and three cages treated with the fungus entomopathogens were used during the tests. The cages were located under the hazelnut canopy and buried to half their height. 200 mature C. nucum larvae were placed in each cage and allowed to burrow naturally. After the larvae had buried themselves, a commercial product was applied to the three treated cages. Mortality was monitored during the following spring. The treated cages showed 99.5% C. nucum mortality as opposed to 63.5% for the untreated cages. These data show the effectiveness of the B. bassiana biological control on key insect pests for hazelnut trees in central Italy. INTRODUCTION Biological control of cultivated plants has become pivotal in phytophagous insect management. This kind of management makes it possible to reduce damage to agroecosystems, avoids insects developing resistance to a number of active ingredients (a.i.), and avoids using another product that would leave residual pesticides. The need to find alternative biological control systems against phytophagous insect pests has led researchers to investigate the possibility of using living organisms against noxious insects in the field, making use of this natural control. These biological controls were used in the present work to control the hazelnut weevil (Curculio nucum L., Coleoptera, Curculionidae) in the area of Monti Cimini (Viterbo, Italy). The Latium region is second most important in Italy for hazelnut production (Corylus avellana L.), being responsible for 29.5% of national production. More than 95% of hazelnut production in the region comes from plantations in the Northern Latium area (Viterbo province) (Carbone et al., 2004) where the weevil is the key insect pest (Paparatti and Pucci, 1987). The adult of the species has the typical rostrum of the weevil family. The female lays one egg in the lower third of the fruit once the fruit has reached a diameter of approximately 11-12 mm. The larva feeds on the seed and, after four instars, leaves the nut by piercing a hole in the pericarp and dropping to the ground. It then burrows down, buries itself, and builds an overwintering chamber in which to hibernate. At the end of spring, adult insects emerge. Traditionally, control of this weevil in the Monti Cimini area has involved the application of two or three chemical treatments per regular period (generally using Lambda-cyhalotrin a.i.). However, chemical treatments have a major effect upon the hazelnut agro-ecosystem, causing a build up of pesticides in nearby Lake Vico. Earlier research identified the biological cycle of the weevil and established a preliminary treatment threshold (Paparatti, 1990; Pucci, 1991, 1992). More recently the adoption of this threshold, as part of guided and integrated pest control (IPM) strategies, Proc. VIth Intl. Congress on Hazelnut Eds.: J. Tous, M. Rovira & A. Romero Acta Hort. 686, ISHS 2005 408 has made it possible to considerably reduce the number of insecticide treatments against weevils. Despite the success of this pest control strategy, it was decided to carry out a preliminary trial with a biological control against C. nucum, using the entomopathogenous fungus Beauveria bassiana (Balsamo) vuill. (Deuteromycotina, Hyphomycetes). B. bassiana was discovered by Bassi in 1835, during the description of a disease that affected silkworms (Bassi, 1836). However, it was only in the 80’s that it developed its toxic action due to various toxic agents produced by the developing fungus (Roberts, 1981). B. bassiana can be found on several substrata (ground, dead insects, bark, etc.) and by 1987, Li had already catalogued over 700 species of arthropod. Several studies have examined the use of B. bassiana as a biological control agent in agriculture (Deseo and Rovesti, 1982; Hluchy and Samsinakova, 1989; Rodriguez and Prassitoli, 1990; Ferron et al., 1991; Nguya, 1993; Puterka et al., 1994; Hsu and Quarles, 1995; Kaaya and Munyinyi, 1995; Quintela and McCoy, 1997; Yasuda et al., 1997; Krueger and Roberts, 1997; Bellotti et al., 1997; Bischoff and Reichmuth, 1997; Moino et al., 1998). In previous research, this entomopathogenic fungus has produced positive results in the control of species from the same insect family as C. nucum (cfr. Adane et al., 1996; Fernandez and Colmenares, 1997; Paparatti and Speranza, 1999; Meikle et al., 2001; Chikwenhere et al., 2001). MATERIALS AND METHODS The experiment was carried out in a biological hazelnut grove situated in the area of Caprarola (Viterbo, Italy) near Lake Vico (620 m above sea level). The hazelnut grove in question is typical of the hazelnut producing area of Northern Latium. The cultivars grown there are the ‘Tonda Gentile Romana’ and the ‘Nocchione’, used for pollination in a ratio of 10%. The plant layout was 5 x 5 m. Sixteen sample plants were randomly chosen for our experiment. On 10 October 1998, the ground below the plant foliage was covered with nets. This made it possible to collect the last instars of the larvae of hazelnut weevil which had left the nuts to bury themselves and hibernate. Ten cubic cages (60 x 60 x 60 cm) were constructed and covered with a thick nylon net (150 g/m, 160 warp-yarns and 60 weft-yarns) a few months before the beginning of the experiment. These cages were buried about 40 cm and then filled with the soil that had been dug out, thereby maintaining the original natural conditions Larvae were collected by hand, twice a day: at around 8 a.m. and around 6 p.m. This frequent collection prevented the larvae stress that could be caused by their staying on the nets and trying to bury themselves. As soon as they had been collected, larvae were put on the soil inside the cages and allowed to bury themselves naturally. Each cage contained 80 weevil larvae. On 10 October 2001, when all the larvae had been buried in the cages, the soil of five cages (treated cages) was treated with a larvicide product; Beauveria bassiana (Bals.)(strain JW-1, ATCC 74040, commercial name Naturalis BioIntrachem Italia) containing 2.3 x 10 living spores per millilitre of product. The recommended dosage was used for the experiment: 12 cc of commercial formula, diluted in 1.5 L of water, evenly soaked around the inner soil layer of the five sample cages. The five sample cages were soaked with an equal quantity of water. On 19 May 2002, the cages were collected and taken to the laboratory where larvae mortality was recorded together with the depth of hibernation. Data regarding larval mortality were collected and elaborated by INSTAT3 software (Graphpad, San Diego, CA, USA). RESULTS The emergence of larvae from the nuts, in the area of Lake Vico is summarised in Fig. 1, which shows its onset from the first ten days in September, with a peak in the second ten days of the same month. As can be seen, the emergence of larvae from nuts followed shortly after (a few days) rainfall at the end of summer. Indeed, more than 75% of larvae were collected in the two or three days following rainfall.