Abstract:
Laboratory studies were conducted to determine the quantities of maize and sorghum stems consumed by larvae of C. partellus and C. orichalcociliel/us as an indication of the amount of damage caused by these species to cereal crops. Results revealed that larvae of C. partellus consumed a greater quantity of maize than C. orichalcociliel/us on a daily basis and throughout the larval lifetime. C. partellus also consumed more sorghum than C. orichalcociliellus on a daily basis, but both species consumed equal amounts of sorghum in their larval lifetimes despite the fact that C. partellus completed a larval generation in a shoter time than C. orichalcociliel/us in both maize and sorghum. This suggests that even ifthe stem borer density at the coastal area of Kenya has not changed since the invasion of C. partellus, damage to cereal crops may be greater than when C. partellus has not invaded. The extent of niche overlap and interspecific association between C. partellus and C. orichalcociliellus were investigated in cultivated and wild host plants. Results showed that C. partellus and C. orichalcociliellus overlapped to a great extent in their habitat, with a very weak positive association at the early larval instars but no association at the late larval instar and pupal stage. No negative association was found between the species suggesting that the species do not directly attack each other while in their habitat. The role of indigenous parasitoids in the population regulation of C. partellus and C. orichalcociliellus was investigated. There was no evidence that indigenous parasitoids are involved in the displacement of C. orichalcociliellus. Investigations on some bio-ecological and physiological aspects of C. partellus and C. orichalcociliellus revealed some superior attributes of C. partellus that may be giving this species a competitive advantage over C. orichalcociliellus. The fecundity, longevity and egg survival of these species were compared in the laboratory at different temperatures and relative humidities. The results showed that both species had similar longevities but C. partellus laid more eggs than C. orichalcociliellus. However, at high temperatures and relative humidities both species had similar fecundity. Higher numbers of C. partellus eggs survived to the first larval instar at all temperatures and relative humidities and this leaves C. partellus with more individuals reaching the reproductive stage. The dispersing abilities of C. partellus and C. orichalcociliellus were compared in the laboratory using a wind tunnel. Higher percentages of C. partellus larvae dispersed from the experimental plants when compared to C. orichalcociliellus in both maize and sorghum plants. Also, C. partellus larvae dispersed over longer distances at a faster rate than C. orichalcociliellus in maize and sorghum plants. Field experiments were conducted to determine the establishment successes of C. partellus and C. orichalcociliellus in maize, sorghum, napier grass and wild sorghum plants. Equal numbers of first instar larvae of C. partellus and C. orichalcoci/iellus were recovered from all plants after 3 days of infestation. However, as the larvae developed, higher numbers of C. partellus larvae were recovered from all the plants except in napier grass, which had higher percentage establishment of C. orichalcociliellus at 10 and 21 days after infestation. C. partel/us and C. orichalcoci/iellus successfully established and coexisted in the same plant at early larval instars, but competition and other mortality causing factors set in at the later instars resulting in the elimination of more C. orichalcociliellus compared to C. partellus. Laboratory studies on the larval developmental times of C. partellus and C. orichalcociliellus in maize, sorghum, napier grass, wild sorghum and guinea grass revealed that C. partellus completed larval development in a shorter time than C. oricha/cociliellus in maize, sorghum and wild sorghum, C. partel/us did not complete development in napier and guinea grasses. On the other hand, C. oricha/cociliellus completed development in all the plants. Development of both stem borer species was faster in maize, much slower in the wild grasses. Survival of both stem borer species was higher in maize, but very low in the wild grasses. The relatively faster development of C. partellus compared to C. oricha/cociliellus in cultivated and wild grasses may be important in the displacement of C. oricha/coci/iellus. However, the ability of C. oricha/cociliellus to complete development in two native grasses in which C. parte/lus did not survive may give this species an advantage over C. partellus. This differential survival in wild grasses may have allowed C. orichalcociliellus to escape extinction from the coast. Investigations on the intensity of diapause and survival of diapausing larvae of C. partellus and C. orichalcociliellus revealed an earlier termination of diapause in C. partellus than in C. orichalcociliellus. C. partellus may be able to colonize crops before C. orichacociliellus, and thus might have an enhanced advantage over C. orichalcociliellus. Male and female moths of diapausing C. partellus larvae tenninated diapause and adults emerged at the same time , whereas, differences were found in the diapause tennination and emergence times of male and female diapausing C. orichalcociliellus larvae. This may facilitate immediate mating and production of viable eggs by C. partellus, while hindering mating of C. orichalcociliellus since the males may not be readily available to fertilize the eggs at the time of female emergence. These superior attributes of C. partellus may be implicated in the displacement of C. orichalcociliellus.