Chilo partellus (Swinhoe) and Chilo orichalcociliellus Strand (Lepidoptera: Crambidae) are gramineous stem borers that occur sympatrically in the southern coastal area of Kenya. Evidence over a 30-yr period indicates that the indigenous stem borer, C. orichalcociliellus, is being gradually displaced by the exotic stem borer, C. partellus. Comparative laboratory studies were conducted in several large-stemmed grasses to examine factors that may be involved in the displacement of C. orichalcociliellus, and to examine other possible effects of the invasion of C. partellus into Kenya. C. partellus had a higher fecundity than C. orichalcociliellus at 25 and 28ЊC, but not at 31ЊC. In addition, more C. partellus than C. orichalcociliellus eggs survived to the Þrst instar. C. partellus larvae developed faster than C. orichalcociliellus in maize and sorghum. In this shorter developmental time, C. partellus consumed more maize than C. orichalcociliellus, but both species consumed equal amounts of sorghum. On a daily basis, C. partellus consumed more maize and sorghum than C. orichalcociliellus. A few C. orichalcociliellus survived to the pupal stage in napier and guinea grasses, whereas no C. partellus survived. The shorter developmental period of C. partellus may give this species a competitive advantage over the slower developing C. orichalcociliellus. However, the ability of C. orichalcociliellus to complete development in two native grasses in which C. partellus did not survive may provide a refuge that has allowed C. orichalcociliellus to escape extirpation from the coastal area of Kenya.

Chilo partellus (Swinhoe) and Chilo orichalcociliellus Strand (Lepidoptera: Crambidae) occur sympatrically on the Kenya coast and are injurious to maize and sorghum. Evidence over a period of 30 years indicates that the indigenous stem borer, C. orichalcociliellus, is being gradually displaced by the exotic stem borer, C. partellus. The two species have overlapping niches, but the specific mechanisms driving the displacement are unknown. The distribution and extent of niche overlap and interspecific association between C. partellus and C. orichalcociliellus were investigated in cultivated and wild host plants. No difference in the within‐plant distribution of the two borer species in maize and sorghum was found. A niche overlap of 83.7% between C. partellus and C. orichalcociliellus was found. A weak positive association was found between small‐, medium‐, and large‐sized larvae of C. partellus and C. orichalcociliellus. However, as larvae grew, the association decreased with no significant association at the pupal stage. No negative association was found between C. partellus and C. orichalcociliellus, suggesting that the two species do not avoid or directly harm each other. The partial displacement of C. orichalcociliellus may therefore, be attributed to intrinsically superior attributes of C. partellus that indirectly affect the survival of C. orichalcociliellus.

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The intensity of diapause of field-collected diapausing larvae of Chilo partellus (Swinhoe) and Chilo orichalcociliellus Strand (Lepidoptera: Pyralidae) was compared in the laboratory at 28 ± 0.5°C under a photoperiod of 12:12 (L:D)h. Chilo partellus terminated diapause faster than C. orichalcociliellus. Mean time to diapause termination was 9.7 days for C. partellus and 14.4 days for C. orichalcociliellus. Male and female C. partellus larvae terminated diapause and adults emerged at the same time, while significant differences were found in diapause termination and in emergence times between male and female C. orichalcociliellus.

Studies on the biology of Acanthomia tomentosicollis (Stal) at Ibadan, Nigeria, showed that the periods of development in the field and the insectary averaged 17 and 18 days, respectively. The hemispherical, hexagonally sculptured eggs were laid in batches of 2-99 (mean-1 eggs per batch in the insectary and 20 in the field). Preoviposition periods were 11-22 days in unmated and 6-9 days in mated females. The mean number of eggs deposited by mated females (202) was significantly higher than that deposited by unmated females (135), although the average oviposition period of 16 weeks in unmated females was three weeks longer than in mated females. Unmated males and females were longer-lived than their mated counterparts, and females generally lived longer than males. These results and other observations on 'the biology of A. tomentosicollis are discussed in the context of its seasonal biology and the pattern of infestation of pigeon pea (Cajanus cajari), its main host plant.