Thermal Requirements and Phenology of the Eucalyptus Snout Beetle Gonipterus Scutellatus Gyllenhal

J. Appl. Entomol. 130(6-7), 368–376 (2006) doi: 10.1111/j.1439-0418.2006.01073.x Ó 2006 The Authors Journal compilation Ó 2006 Blackwell Verlag, Berlin Thermal requirements and phenology of the Eucalyptus snout beetle Gonipterus scutellatus Gyllenhal S. Santolamazza-Carbone, A. Rodrı´guez-Illamola and A. Cordero Rivera Grupo de Ecoloxıá Evolutiva e da Conservacioń, Departamento de Ecoloxıá e Bioloxıá Animal, Universidade de Vigo, Pontevedra, Spain Ms. received: January 28, 2006; accepted: May 11, 2006 Abstract: Laboratory experiments and field surveys were carried out to study the thermal requirements and phenology of the Eucalyptus snout beetle Gonipterus scutellatus (Curculionidae) and its parasitoid, Anaphes nitens (Mymaridae). Developmental times were recorded for G. scutellatus life stages: egg to first instar larva, first instar to pre-pupal larva, prepupae to adults and the complete life cycle. Experiments were performed in temperature-controlled chambers maintained at 10, 15, 20, 25 and 30°C with a photoperiod of 11 : 13 h of light : darkness and 50–60% RH. To calculate the minimum threshold temperature of the parasitoid, parasitized egg capsules were kept under similar conditions. During 1998 and 1999 we studied the phenology and the day-degree (DD) accumulation of G. scutellatus and its parasitoid in plots of Eucalyptus globulus at six different sites in NW Spain. Every 2 weeks, the numbers of snout beetle adults and egg capsules were counted in each plot. The rate of parasitism was estimated by collecting 90 egg capsules from each plot on each sampling date. We recorded the temperatures in each plot to test whether differences in temperature alone could account for the phenology of this snout beetle. To complete a full life cycle from egg to adult, the weevil required a mean of 1119.83 ± 20.59 DD above a base temperature of 6.11°C. The parasitoid had a base temperature of 5.09°C and needed 318.16 DD to complete a life cycle. Our model indicated that three generations of snout beetle could develop each year, corresponding to peaks of snout beetle numbers in the field in March–April, June–July and November. In some years only one generation of G. scutellatus was recorded due probably to the effectiveness of the parasitoid. Differences in numbers of adults and egg capsule were recorded between neighbouring Ôcoastal plotsÕ and between neighbouring Ôinland plotsÕ. Hence, climate alone does not appear to explain the phenology of G. scutellatus. Key words: Anaphes nitens, Eucalyptus globulus, day-degree, developmental thresholds, host–parasitoid interactions, pest management 1 Introduction Eucalyptus. Both the snout beetle adults and larvae eat the leaves, buds and shoots of the Eucalyptus trees, Predicting the timing of particular stages in the life which retards tree growth and contorts and eventually cycle of pest insects is important in studies of their kills branches of trees that are heavily infested (Tooke, population dynamics and for forecasting pest insect 1955). Female snout beetles lay their hard brown egg attacks in cultivated crops (Nylin, 2001). The devel- capsules on shoots and young leaves. The egg capsules, opment of ectotherm organisms occurs within a composed mainly of faeces, contain about eight eggs. narrow range of temperatures, and this has a profound The neonate larvae emerge after 7–10 days and the effect on all aspects of their development. Physiological pass through four instars. The first instars feed on the time (PhT) is the amount of heat required over time for surface of the leaves, whereas the later instars consume an insect to complete a full life cycle or simply to the entire leaf blade. This snout beetle and its complete one specific stage of development (Taylor, parasitoid, A. nitens, have been studied since the start 1981). A day-degree (DD) is the amount of heat that of the 20th century primarily because of a combination accumulates above a specific base temperature during of the high pest status of the snout beetle and the good each 24-h period (Baskerville and Emin, 1968). This possibility of controlling it with the above parasitoid work provides the first detailed analysis of the thermal (Marelli, 1928; Tooke, 1955; Arzone and Vidano, requirements and phenology of the Eucalyptus snout 1978; Mansilla, 1992; Cordero Rivera et al., 1999; beetle Gonipterus scutellatus Gyllenhal (Col., Curculi- Hanks et al., 2000). onidae) and one of its natural enemies, the parasitoid The Eucalyptus snout beetle was introduced accident- Anaphes nitens Girault (Hym., Mymaridae). This snout ally into Galicia, NW Spain in 1991. The egg parasitoid beetle is of Australasian origin and feeds specifically on Thermal requirements and phenology of G. scutellatus 369 A. nitens was released towards the end of 1993 as a daily. To study the developmental time and thermal require- biological agent for controlling the pest and gave ments of the parasitoid A. nitens, we used egg capsules that promising results (Mansilla and Pe´rez Otero, 1996). were both laid and parasitized under laboratory conditions. However, following the initial success, control became Over the range of temperatures that are suitable for variable due to periodic fluctuations in the populations development, the relationship between the rate of development of a given insect and temperature is approximately of both the pest and its parasitoid (Santolamazza linear (Campbell et al., 1974). To estimate PhT, one of the Carbone 2002). Despite the economic importance of important parameters is the determination of the minimum this snout beetle, little information is currently available threshold temperature (MTT), the temperature above which on the thermal requirements and phenology of either development can start. The MTT is usually obtained by G. scutellatus or A. nitens in the field. Depending upon plotting the rate of development against temperature. The the country chosen and its climate, this weevil may either point where the projected line intersects the horizontal be restricted to one generation per year or breed temperature axis is taken as the MTT. PhT measured in continuously (Clark, 1931; Moutia and Vinson, 1945; DD can be calculated using the equation: Tooke, 1955; Arzone and Meotto, 1978; Mansilla and PhT ¼ tðTm À MTTÞ; Pe´rez Otero, 1996). In an earlier study in NW Spain we recorded only one generation of snout beetles in 1996 where t is the development time in days and Tm is the mean and three in 1997, as in 1996 the parasitoid gave almost treatment temperature. To record the time required for egg development of 100% control and this resulted in a local extinction of G. scutellatus, 50 fresh egg capsules (mean egg capsule the pest (Cordero Rivera et al., 1999). However, this size ¼ 8.14 ± 0.09 eggs) were placed individually into plastic, conclusion was based on data collected from only one 10 cm diameter, Petri dishes. Ten Petri dishes of eggs were then plot and so needs to be verified by collecting a more placed into each of five temperature-controlled chambers robust set of data. maintained at 10, 15, 20, 25 and 30°C, and having a Knowledge of developmental times and phenology photoperiod of 11 : 13 h of light : darkness and 50–60% of pest insects in the field should enable to predict the RH. The egg capsules were checked daily and any larvae that timings of pest insect attacks and so improve pest had hatched were transferred to plastic boxes before being control (Gimeno Sevilla and Perdiguer Brun, 1993; returned to their respective chambers. To obtain a more precise Thomas, 1997; Milonas et al., 2001; Lobinske et al., estimate of the temperatures within the experimental cham- 2002). bers, each chamber contained a data logger that recorded the temperature every hour. Each day the larvae were provided We tested whether data from laboratory experiments with fresh leaves of E. globulus as food. However, the leaves and field samples could be used to construct a DD soon dried out in the 30°C chamber and so the leaves were model that would help make more efficient the current renewed twice daily in this treatment. Despite the additional Integrated Pest Management (IPM) programme in the food, larval mortality was high at 30°C and few individuals NW Spain (Santolamazza Carbone and Fernańdez de survived the treatment. The smaller larvae, instars 1–3, were Ana Magań, 2004). kept in 10 · 15 cm plastic boxes in groups of up to 10 Our aims in this study were to: (i) estimate the individuals. When larvae reached the fourth instar (pre-pupal developmental thresholds and DD requirements of stage) they were transferred to larger, 1 l boxes that contained the snout beetle and its natural enemy, (ii) study the 10 cm of sterile soil to allow the prepupae to burrow into the phenology of the two insects in six field plots over a soil prior to pupation. The date each prepupa started to burrow was recorded, and we then assumed that the first to 2-year period, and (iii) test the accuracy of a DD model burrow corresponded to the first adult to emerge from the soil. for predicting the phenology of G. scutellatus. In this way we recorded the duration of development of each Our hypothesis is that if the phenology of G. scu- individual. We also recorded the mean date of burrowing and tellatus is controlled mainly by temperature, then the the mean date of emergence at each temperature and used the phenology of this insect should be similar in localities difference as the estimate of mean development time. As both with similar climates. In contrast, if the biological methods gave similar values, only the second estimate is control agent proved to be the key mortality factor, presented in this study.

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