PERFORMANCE OF STERILIZED ELDANA SACCHARINA WALKER (LEPIDOPTERA: PYRALIDAE) IN MATING TRIALS

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1 SHORT, NON-REFEREED PAPER PERFORMANCE OF STERILIZED ELDANA SACCHARINA WALKER (LEPIDOPTERA: PYRALIDAE) IN MATING TRIALS MUDAVANHU P 1, CONLONG DE 1,2 AND ADDISON P 1 1 Department of Conservation Ecology and Entomology, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa 2 South African Sugarcane Research Institute, Private Bag X02, Mount Edgecombe, 4300, South Africa mudavanhu@sun.ac.za prideshadreck@yahoo.co.uk Abstract At low doses of radiation F1 sterility of the sugarcane stalk borer Eldana saccharina can be attained. This bodes well for the progression of Sterile Insect Technique (SIT) against this economically important pest. The mating behaviour of E. saccharina was investigated in still-air cage trials using video technology to determine whether mass-rearing and irradiation of adult moths altered their performance in lek formation and mating success compared to their wild counterparts. Mating frequency, calling frequency, mating duration, onset of mating and onset of calling were used to make comparisons and assessments in the bioassay. Results from this study indicated that males irradiated at 150 Gy are as competitive as wild males in mating behaviour. Mating combinations comprising both labreared male and female moths had higher mating frequencies than those comprising wild moths. There were no significant differences in mating performance between moths irradiated at 150 Gy and those irradiated at 200 Gy. Positive results from these trials indicate potential for the development of an effective commercial SIT approach in the integrated pest management programme against E. saccharina. Keywords: Eldana saccharina, area-wide IPM, sterile insect technique, mating behavior, insect quality, mass-rearing Introduction The sugarcane stalk borer, Eldana saccharina Walker (Lepidoptera: Pyralidae) (eldana) is the most destructive pest and most limiting factor in the South African sugar industry, with current crop losses estimated at R60 million/annum (Conlong, pers. comm. 1 ). The use of sterile insect technique (SIT) is one of the newer methodologies being developed as part of an Area-Wide Integrated Pest Management (AW-IPM) strategy against this key pest. The SIT is a strategy that imposes birth control on the pest population to further reduce its numbers (Klassen and Curtis, 2005). According to Robinson (2005) this method involves mass rearing of the target species, exposing them to gamma rays to induce sterility and then releasing them into the target population. Theoretically, the released sterile males will then mate with wild females thereby preventing them from reproducing, and consequently an entire generation would never see the light of day. 1 DE Conlong, South African Sugarcane Research Institute, Mount Edgecombe, 4300, South Africa. 287

2 There is considerable scope for developing efficient eldana SIT and one way of achieving this is improving sterile male performance through a better understanding of the mating behaviour of this insect (Hendrichs et al., 2002). A major concern for entomologists and managers of AW-IPM programmes that use SIT is that released sterile males compete well with wild males and successfully mate and inseminate the targeted wild females (Calkins and Parker, 2005). Sterile insect quality can be assured through a system of bioassays of quality parameters that reflect the insect s ability to survive, interact with its environment, locate, mate with and fertilise females of the target population (Calkins and Parker, 2005). Mass-rearing is necessary to produce large quantities of males required for SIT, but the lab environment imposes unnatural selection pressures to which a population will adapt, resulting in increased fitness for the new environment and enabling the lab population to persist (Iwahashi, 1996; Matos et al., 2000). The strong laboratory selection may result in the alteration or loss of ecological and behavioral traits that are necessary for mass-reared males to remain competitive when released in the field (Saul and McCombs 1995; Iwahashi 1996; Dalby-Ball and Meats 2000; Briceno and Eberhard, 2002), which is further compounded by irradiation (Weldon, 2005). In this study we developed a series of tests to confirm that these behavioral traits are not lost in the mass-reared and sterilized insects. The mating behaviour of wild, mass-reared (lab-domesticated) and sterile (labdomesticated, gamma irradiated) male eldana with females from different sources (i.e. wild, mass-reared, irradiated) was studied. We tested the hypothesis that lab-domesticated and gamma-irradiated eldana moths are as competitive in lek formation and mating as wild eldana moths. Materials and Methods Wild eldana moths were collected as pupae from various sugarcane plantations in the KwaZulu-Natal Province and stored in multi-cell eclosion trays in incubators at 26 C, 10:14 Light: Dark (L:D) cycle. Mass-reared eldana were obtained as pupae from the massrearing facility at the South African Sugarcane Research Institute at Mount Edgecombe, Durban. The mass-reared moths were marked by addition of Calco Red to their larval diet, a tool that distinguishes them from wild moths. The live insects were couriered as pupae to the Conservation Ecology and Entomology Department at Stellenbosch University in the Western Cape Province, where all the experiments were conducted. Sterile moths were produced from the lab-reared population by irradiating them soon after eclosion with gamma-radiation from a cobalt-60 source in normal atmosphere at the SIT-Africa Radiation facility in Stellenbosch. A ventilated Perspex still-air cage (300 x 300 x 300 mm) with a transparent lid and used as mating arena was installed in a temperature and light controlled room. A Logitech Pro 9000 webcam to record video footage of moth activity was placed above the mating arena and illuminated by a 100 watt Phillips infrared light bulb, since eldana moths mate during the dark (Atkinson, 1981; Zaggatti, 1981). Only freshly eclosed (0-day old) virgin moths were used for the bioassays. Each treatment consisted of an equal sex ratio of 10 males and 10 females confined in one mating arena and observed continuously for a maximum of two nights. The moths were maintained and studied under a 10:14 L: D-cycle at 26±1 C and 60-80% RH. The following mating treatments were used: (i) (ii) lab-reared males and wild females (LR) 200 Gy sterile males and wild females (ST200M) 288

3 (iii) 200 Gy sterile females and wild males (ST200F) (iv) wild males and females (Control) (W) (v) 150 Gy sterile males and wild females (ST150M) (vi) lab-reared males and females (MX) (vii) 150 Gy both sexes sterile (ST150MF) (viii) 200 Gy both sexes sterile (ST200MF). Five replicates were performed for each treatment. The following measurements were recorded in order to assess, compare and evaluate moth quality: (i) (ii) (iii) (iv) mean onset of calling mean onset of mating mating frequency mean duration of copulation. Although the lab-reared and sterilized moths were marked with Calco Red or fluorescent dye, it was not possible to see the marks or distinguish the individual moths in the arena under infrared illumination. For this reason all measurements are collective descriptors of a group of 10 males and 10 females. Weldon (2005) states that, although this masks individual variation among males or females, it still enables an overall assessment of variation among the different strains or treatments. The data was analysed using a one-way ANOVA of the averages of each quality parameter per cage. Univariate Tests of Significance for the square root of totals (averaged for the two nights/scotophases) were conducted in STATISTICA, and the averages for the first and second night were used as the dependent variable. Results Mating frequency There were significant differences in mating frequency between some treatments (P<0.005, F= , df=7) (Figure 1.). Wild males (W) had the lowest mating frequency while males and females irradiated at 150 Gy (ST150MF) had the highest. There was no sufficient statistical evidence suggesting that irradiated males were significantly different from wild males in mating frequency with wild females (W, ST150M and ST200M). Treatments employing both lab-domesticated males and females (MX, ST150MF and ST200MF) had higher mating frequencies than those treatments involving either wild males or females or both (i.e. W, LR, ST150M, ST200M and ST200F). Onset of calling and mating duration There were significant differences between the treatments in the mean onset time of calling after lights-off (Kruskal-Wallis test: H (7, N=40) = , P=0.0008). There were no significant differences in mating duration averages between the treatments (Kruskal-Wallis test: H (7, N=39) = 7.327, P=0.3957). The control treatment (W) had the shortest mating duration (1 h 33 min 38 s) while males irradiated at 200 Gy had the longest (2 h 39 min 1 s). Sterile males irradiated at 150 Gy had the earliest onset times of calling and mating after lights-off (00:36:29 and 01:09:46 289

4 respectively), while wild males had the latest onset times (09:14:13 and 09:21:18 respectively). Figure 1. Frequency of matings by the different treatments (means: square root of total matings averaged over two scotophases). Discussion and Conclusion The observations from this study are consistent with findings by Weldon (2005) who demonstrated that mass-rearing leads to a departure of male mating behaviour away from that exhibited by wild males. Lab-domestication extended the duration of male mating behaviour of eldana. This was also observed with mass reared Bactrocera tryoni when compared to their wild counterparts. Kuba and Koyama (1982) attributed this to genetic differentiation in the mass-reared strain, while Lux et al. (2002) states that sterilisation further compounds this behaviour. This could be the explanation for steriles and lab-reared moths having higher mating frequencies and initiating calling and mating earlier than the wild strain. The decrease in mating frequency of males irradiated at 200 Gy may be indicative of a decrease in sexual motivation and overall mating performance as a result of increasing the irradiation dose from 150 Gy. The data therefore suggest that irradiated males are as competitive as wild males in mating behaviour in laboratory bioassays and hence could be ideal for use in the sterile release program. This however needs verification under field conditions. An irradiation dose of 150 Gy is more desirable than 200 Gy. REFERENCES Atkinson PR (1981). Mating behaviour and activity patterns of Eldana saccharina (Walker) (Lepidoptera: Pyralidae). Journal of the Entomological Society of Southern Africa 44(2):

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