Niche overlap and interspecific association between Chilo partellus and Chilo orichalcociliellus on the Kenya coast

Chilo partellus (Swinhoe) and Chilo orichalcociliellus Strand (Lepidoptera: Crambidae) occur sympatrically on the Kenya coast and are injurious to mai ze and sorghum. Evidence over a period of 30 years indicates that the indig enous stem borer, C. orichalcociliellus, is being gradually displaced by th e exotic stem borer, C. partellus. The two species have overlapping niches, but the specific mechanisms driving the displacement are unknown. The dist ribution 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% bet ween C. partellus and C. orichalcociliellus was found. A weak positive asso ciation was found between small-, medium-, and large-sized larvae of C. par tellus and C. orichalcociliellus. However, as larvae grew, the association decreased with no significant association at the pupal stage. No negative a ssociation was found between C. partellus and C. orichalcociliellus, sugges ting that the two species do not avoid or directly harm each other. The par tial displacement of C. orichalcociliellus may therefore, be attributed to intrinsically superior attributes of C. partellus that indirectly affect th e survival of C. orichalcociliellus.