Abstract:
Radio-sterilization study was conducted on the biology of two lepidopterous maize stem
borers, Eldana saccharina Walker and Sesamia calamistis Hampson to induce inherited
sterility for insect pest management programme in Ghana. Bioecology of the two borer
species was also studied for a 3-year period, 1997-1999 at Medie, a predominantly maize
growing community in the Ga District of Greater Accra Region, to determine the
population dynamics and climatic factors influencing the population of the borer species.
It was observed that Sesamia species usually attacked the young maize crop, with peak
infestations occurring about 6-8 weeks after emergence of the crop. Eldana species
preferred mature maize with peak infestations around 10-12 weeks after emergence of the
maize crop. Larval numbers of E. saccharina showed inverse relations with rainfall, r= 0.5899; p= 0.043. Infestation levels of larvae of both species were relatively higher
during the minor rainy season than the major season. Larvae and pupae of E. saccharina
that developed separately on natural and artificial diets did not show significant
difference in most of their biological parameters. In both borer species, pupal weights of
natural dieters were slightly heavier than those of artificial dieters. Exposure of young
pupae (less than 6 days old) of the two borer species to increasing doses of ionizing
radiation, 80-180 Gy, resulted in high percentage of deformity and unemerged adults.
Mature pupae of 6-8 days old were less susceptible to increased doses and exhibited
fewer body deformities and unemerged adults. The mating capability of adults emerged
from irradiated mature pupae of E. saccharina was not adversely affected. In the parent
generation (P), fecundity and fertility decreased with increased doses of radiation for crosses involving irradiated males and normal females as well as irradiated females and
normal males. While the treatment in the former crosses resulted in partial fertility (400!o
at 180 Gy), the latter produced almost sterile individuals (96%, with doses above 100
Gy). In the F) generation, both fecundity and fertility were inversely related to increased
doses of radiation. Crosses involving F) males showed lower fertility than their
corresponding F) females. Fertility was reduced to l00!o with radiation dose of 100 Gy in
the former, whilst similar reduction in F) females required about 150 Gy or more. The
overall sex ratio of emerged adults in F) and F2 generations shifted in favour of males.
Thirty pairs of chromosomes were observed in normal meoitic gametes (metaphase 1) of
E. saccharina. Chromosomal aberrations resulting from induced radiations were found
in the form of fragments, rings and chains. In experimental field cages, moths treated
with radiation dose of 150 Gy, resulted in decrease in fecundity and fertility of F)
generation when the ratios of sterile to fertile moths was increased. Fecundity was
reduced by 47% with ratio of 1 sterile to 1 fertile and 79% with mating ratio of 5 sterile
to 1 fertile, as compared with the control of 1 fertile to 1 fertile. Since significant
reduction in fecundity and fertility was 0 bserved with radiation doses between 120-180
Gy, it is suggested that this range could be used to induce inherited sterility in E.
saccharina. The substantial reduction of population of the borer species due to induced
sterility in the progeny implies that inherited sterility could be used as a control strategy
to suppress the stem borer populations in insect pest management programme in Ghana.
