Gahan) [Hym.: Scelionidae], an Egg Parasitoid of Sesamia Calamistis Hampson (Lep.: Noctuidae
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BIOLOGICAL CONTROL 8, 15–21 (1997) ARTICLE NO. BC960478 Factors Influencing the Developmental Rates and Reproductive Potentials of Telenomus busseolae (Gahan) [Hym.: Scelionidae], an Egg Parasitoid of Sesamia calamistis Hampson (Lep.: Noctuidae) A. CHABI OLAYE,* F. SCHULTHESS,* T. G. SHANOWER,† AND N. A. BOSQUE-PE´ REZ‡ *Plant Health Management Division (PHMD), International Institute of Tropical Agriculture (IITA), Biological Control Center of Africa, BP 08-0932 Cotonou, Republic of Benin; †International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru P.O., Andhra Pradesh, 502 324, India; and ‡PHMD, International Institute of Tropical Agriculture (IITA), PMB 5320 Ibadan, Nigeria Received September 25, 1995; accepted July 18, 1996 (Bosque-Pe´rez and Mareck, 1990; Shanower et al., The effect of seven constant temperatures on the 1991; Gounou et al., 1994). Damaging population levels development of Telenomus busseolae was determined, are usually reached on second season maize, between using Sesamia calamistis eggs as the host. The develop- September and November. In the Republic of Benin, S. mental threshold calculated was 13.7°C, the optimum calamistis is the most abundant borer species but temperature was 31°C, and the maximum temperature population densities usually are below injury level at which no parasitoid emergence occurred was 34°C. (Shanower et al., 1991), although multiple cropping Female T. busseolae began ovipositing immediately cycles of maize in southern Benin theoretically should after emergence. They produced more offspring dur- cause serious yield losses in late planted maize. The ing the first 24 h of adult life than during any subse- reasons for these generally low S. calamistis densities quent period. At 20°C, adult females lived twice as long in Benin are unknown. Recent studies (Chabi Olaye, 1992) as those at 30°C (21.7 and 11.0 days, respectively). Total progeny of T. busseolae was significantly highest at and surveys (Se´tamou and Schulthess, 1995) have shown 27°C. Mean fecundity ranged from 61 to 182 offspring that egg parasitism plays an important role. In southern per female. The effect of host age and host deprivation Benin, the most common egg parasitoids are Telenomus on the host parasitism rate, egg viability, and sex ratio busseolae (Gahan) followed by T. isis Polaszek [Hym.: of T. busseolae was also investigated. Only parasitism Scelionidae], accounting for more than 90% of all species and emergence rates were affected by host age. The found. A survey in 65 farmers’ fields in 1993 revealed an numbers of total progeny were the same between 0 and average rate of parasitism of 76.4% by both species. Teleno- 10 days of host deprivation, whereas longevity tended mus spp. were shown to be one of the most important to increase from 12.8 to 23.4 days from 0 to 14 days of factors suppressing S. calamistis densities and reducing withholding hosts. The percentage parasitoid emer- damage to the plant (Se´tamou and Schulthess, 1995). gence and the sex ratio were not affected by withhold- In subsaharan Africa, T. busseolae is frequently ing hosts. T. busseolae is adapted to an ecosystem with found associated with lepidopteran stemborers, and strong environmental fluctuations that cause tempo- particularly Sesamia spp. and Busseola fusca Fuller rary scarcity of hosts and food. r 1997 Academic Press (Lep.: Noctuidae) (Polaszek and Kimani, 1990). How- KEY WORDS: Telenomus busseolae; Sesamia calamis- ever, information on temporal and spatial distribution, tis; life table statistics; temperature; host age; host as well as on the biotic potential, and thus importance, deprivation; West Africa. of this species are scarce. The present study was carried out to provide information on the bionomics of T. busseolae as affected by temperature, age of host eggs, INTRODUCTION and host deprivation to help to explain fluctuations in the population dynamics of this species in the field. Across Africa, the most serious insect pests of field- grown maize are lepidopteran stem and earborers belonging to the families Noctuidae and Pyralidae MATERIALS AND METHODS (Harris, 1962; Atkinson, 1980). In West Africa, Sesamia Host and Parasitoid Culture calamistis Hampson (Lep.: Noctuidae) and Eldana saccharina Walker (Lep.: Pyralidae) are the predomi- Larvae of S. calamistis were reared on ears of maize nant species in areas with bimodal rainfall distribution in the soft dough stage, in sterile plastic jars (13.6 15 1049-9644/97 $25.00 Copyright r 1997 by Academic Press All rights of reproduction in any form reserved. 16 OLAYE ET AL. length 3 11.3 cm diameter). The ears were soaked in a Effect of Host Age and Host Deprivation on the 10% bleach solution for 15 min and then rinsed with Reproductive Potential of T. busseolae sterilized water and dried with paper towel. Four ears To determine the effect of host age on the reproduc- with both ends cut off were placed in a jar with 200 tive potential of T. busseolae, 100 S. calamistis eggs of 1-day-old larvae. The ears were changed once a week, desired age (0, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or 5 days old) and on Week 2 the number of larvae per jar was were exposed to previously mated females in transpar- reduced to 100. After 3 weeks pupae were extracted and ent vials at 27 6 1°C. Each treatment (host age level) transferred to oviposition cages for adult emergence. was replicated 10 times. After 6 h exposure time, the Moths were handled according to the method of Bosque- eggs were transferred to new vials for incubation. Pe´rez and Dabrowski (1989). For the first host deprivation studies, adult parasi- T. busseolae adults were obtained from a culture toids were kept with honey but without hosts for 2, 6, established at IITA Station, Benin. These parasitoids 10, 14, 20, and 30 days at 27 6 1°C. Newly emerged and were obtained from collections in maize fields in south- mated females were used as controls (0 days of host ern Benin the same year and were reared on eggs of S. deprivation). From each of the seven groups, 10 mated calamistis in transparent vials (11.6 length 3 4.5 cm females were individually isolated in transparent vials diameter) with honey streaked on the inside surface to together with 100 host eggs. After 6 h exposure time, provide food for adults. Fresh egg batches were sup- eggs were removed and transferred to an incubator. plied daily until death of adult T. busseolae. Because In a second host deprivation study, adult parasitoids this parasitoid can easily be mistaken for T. isis Polas- were kept with honey but without hosts for 0, 2, 6, 10, zek, specimens are frequently reidentified using the and 14 days at 27 6 1°C. Mated females of the five age Polaszek et al. (1993) key. groups were individually isolated in transparent vials together with 100 host eggs. Every 24 h, old eggs were Effect of Constant Temperatures on the Bionomics removed and incubated, and 100 new eggs were pro- of T. busseolae vided until death of the female. Each treatment was replicated 10 times. The effect of temperature on the developmental rate The rate of parasitism was estimated as the number of T. busseolae was studied under eight constant tem- of emerged parasitoids plus clearly parasitized but peratures: 15, 18, 20, 25, 27, 30, 32, and 34°C. Depend- unhatched eggs over total number of eggs exposed. ing on temperature, the relative humidity ranged be- Rate of emergence was determined by dividing the tween 60 and 99%. Egg batches of 20 1-day-old eggs number of emerged adults by the number of visibly were exposed to one female parasitoid per treatment. parasitized eggs. The sex ratio of the offspring was Each treatment was replicated three times. After 6 h calculated as the proportion of females on total num- exposure, the eggs were transferred to new vials and bers of adults. kept under each selected temperature for incubation. The egg masses were observed twice daily until para- Statistical Analysis sitoid emergence. Developmental time was deter- mined from the end of the 6-h oviposition period to The nonlinear model described by Logan et al. (1976) emergence. was used to predict development of T. busseolae. Hence, For assessment of fecundity and longevity of adults, the relationship between the developmental rate per egg masses were exposed to parasitoids in incubators day Z[Tc] and the temperature Tc is given by for6hat20,25,27,and30°C constant temperature. [ p2(Tc2T0)] [ p2(Tm2T0)21/p3(Tm2Tc)] Approximately 2 h after parasitoid emergence, 20 pairs Z[Tc] 5 p15e 2 e 6, [1] were selected per temperature treatment. Each pair was placed in a separate oviposition unit which con- where T0 is the lowest experimental temperature and tained batches of 100 1-day-old S. calamistis eggs (this Tm represents the maximum temperature at which no stage and number of eggs were found to be adequate in parasitoid emergence occurred in the present experi- a preliminary experiment) and honey. New eggs were ment. The parameters p1, p2, and p3 were estimated by provided every 24 h, and the number of T. busseolae Marquardt’s (1963) nonlinear least squares technique females still alive was recorded daily until death. using the SPSS program (Norusis, 1990). The calcula- Exposed egg batches were held under the previously tion of the residual sum of squares was weighted with described temperatures until parasitoid emergence. the number of individuals surviving at each tempera- Parasitoids were counted and sexed. Longevity and ture. The model was validated using development rates fecundity of adult parasitoids and sex ratio of offspring calculated for each temperature. The Logan et al.