Host-Plant Specificity in the Cactophilic Drosophila mulleri Species Complex

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Abstract
(1) Jaenike''s model of optimal oviposition behaviour in phytophagous insects was tested in a guild of four closely-related cactophilic Drosophila species, namely D. mojavensis, D. arizonensis, D. aldrichi and the undescribed form ''from Navojoa''. (2) Rearing records from nearly 300 cactus rots collected in Southern Sonora and Northern Sinaloa (NW Mexico) showed a sharp niche separation between the species breeding on Opuntia (prickly pear) and those breeding on Stenocereus (columnar cacti). (3) The suitabilities of four different host-plants were assessed by measuring three fitness traits (viability, development time and thorax size) on cactus pieces fermented in the laboratory with natural yeasts and bacteria. The results are in good agreement (with one exception) with the expected predictions of the model in that the resident species had higher overall fitness on its own host-plant. These data support the notion that differences in plant suitability underlie feeding specialization. (4) The shift in host-plant use from opuntias to columnars during the evolution of these flies must have entailed a loss of fitness which could have occurred in isolation under stressful environmental conditions.