A technique for the laboratory rearing of the cluster caterpillar, Spodoptera litura (Fabricius), based on a modified Shorey and Hale (1965) larval medium is described, and the problem of diseases in the culture and methods of sterilization are discussed.
The biology of S.litura was studied in the laboratory and the time required for the development of adults from eggs was found to be 23.4±0.4 days. Adult females lived 8.3±0.6 days and laid 2,673±148.7 eggs during a period of 8 days. The mean longevity of males was 10.4±0.9 days.
The mating behaviour of S.litura moths was investigated, and it was found that moths did not mate during the night of emergence, but mating did occur on the first night after emergence. Generally, males and females mated only once per night. The highest mean frequency of mating during a lifetime was 10.3 for males, and 3.1 for females.
Examination of the morphology of the male and female reproductive systems revealed distinct differences between S.litura and other noctuid species so far studied and revealed that the female S.litura reproductive system is less complex than the three noctuids examined by Callahan (1958) and Callahan and Chapin (1960).
Sperm type, quantity and transfer were examined. The two types of sperm known to occur in Lepidoptera were identified, described and quantity of each transferred to females during mating was determined. Untreated males transferred approximately 1,052,640 sperm (equal amounts of apyrene and eupyrene) per mating. Apyrene sperm in S.litura are longer than eupyrene sperm and both types of sperm enter the utriculus lobe of the spermatheca of the female and persist there for the entire lifetime of the female. No sperm were found in the lagena.
From reciprocal matings of irradiated and untreated males and females it was found that adult males were more radioresistant than adult females. A dose of 35 krad to newly emerged males caused 99 percent sterility in terms of number of larvae produced per female and was thus chosen as the sterilizing dose, while a dose of 50 krad caused 100 percent sterility. Females were sterilized (99.2 percent) with 15 krad and completely sterilized with 25 krad.
Irradiation of male and female S.litura with doses up to 60 krad did not significantly reduce their mating frequency and had no deleterious effect on survival.
Sub-sterile males (15 krad) were more competitive than sterile males (35 krad), and females mated to 15 krad males contained more eupyrene sperm than did females mated to 35 krad males. The number of apyrene sperm was not significantly affected.
The phenomenon of inherited sterility was investigated. When parental male S.litura were irradiated with 10, 15 or 20 krad, the developmental time of the F1 immature stages generally increased as the dose of irradiation increased. Percentage malformation and mortality among the immature stages, and percentage malformation among emerging adults also increased with increasing irradiation dose. The sex ratio of the F1 generation was distorted in favour of males, and this distortion was dose dependent.
F1 progeny, whether male or female, were more sterile than their irradiated male parent except F1 females from the 20 krad dose which were slightly less sterile than the P males. F1 male progeny of male parents irradiated with 15 or 20 krad were 98.30 and 99.99 percent sterile, while F1 female progeny were 98.0 and 78.9 percent sterile respectively.
When 15 krad males were released in laboratory cages at 4:1:1 and 9:1:1 (irradiated males:untreated males:untreated females) ratios, the reduction of the number of F1 larvae was 51.6 and 50.8 percent respectively. Release of 35 krad males at the same ratios gave 50.5 and 79.1 percent reduction compared with the control.
When untreated female S.litura were alternately mated with irradiated and untreated males, the second mating was the effective mating with more than 81 percent of the females, while approximately 19 percent of the females had egg hatch similar to that of the first mating (suggesting that in these females the first mating was the effective mating). Examination of sperm transfer after one and two matings of untreated female S.litura with untreated males showed clearly that females mating a second time started to empty their spermathecae during the second mating, and that after 30-40 min after termination of the second mating the spermathecae were devoid of sperm from the first mating.
It was also found that the mating propensity of sterilized males was less than that of untreated and sub-sterilized males; and that untreated males remained in copula for longer periods than irradiated males.
Laboratory reared males performed satisfactorily under field conditions and there was no indication of recovery of fertility in irradiated males during the 8-day period of the study.
Hypothetical models for population suppression of a natural population of S.titura were erected utilizing as much of the laboratory data obtained as was possible. The results of the model projections showed that a single release of sub-sterile (i.e. 15 krad) males resulted in lower F2 population than did a single release of sterile (i.e. 35 krad) males. Two releases of 35 krad males gave a lower F2 population than did the single release of 15 krad males, but in terms of control compared with the untreated population the difference was not great-98.4 percent for 35 krad and 96.0 percent for 15 krad males. Considering that the cost of mass rearing Lepidoptera is much higher than the cost for other insects, the fact that one release is indicated to be sufficient to control two generations presents a substantial saving in the cost of a programme. The models demonstrated the advantage of utilizing inherited sterility arising from the release of sub-sterile males for population suppression of S.litura over the release of sterile males.