Allopatric populations sometimes display reproductive barriers in incipient form, and may thus reveal potential mechanisms of speciation. We conducted mating trials to estimate the degree of precopulatory (behavioral) and postcopulatory (gametic) prezygotic isolation between African and Asian populations of the seed beetle Callosobruchus maculatus. The populations differ in several fitness-related traits, and have been associated with different legume hosts. In single-pair trials, the probability of copulation within 10 min was lower when Asian females were paired with African males than it was in the other three pairing combinations. This pairing also provided evidence for gametic isolation, as Asian females mated once to African males laid fewer eggs than did females in the other treatments. Variation in fecundity could not be explained by the duration of copulation, which did not differ among pairing combinations. There was also no effect of pairing combination on the number of copulation attempts, latency to copulation, or rate of egg hatch. When females were simultaneously presented a male from each population, Asian males obtained a disproportionate share of the matings, and were more likely to disturb a pair already in copulo. Surprisingly, male size did not influence the probability of copulation in the single-male trials, the outcome of male–male competition, or the fecundity of once-mated females. Although the African and Asian populations showed some evidence of mating incompatibility, the absence of symmetrical isolating factors suggests minimal barriers to gene flow.

Several resistant lines of wheat, Triticum aestivum (L.) (Poaceae), have been developed to control populations of the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae). We examined the potential effects of D. noxia resistant lines on another aphid, the bird cherry-oat aphid (Rhopalosiphum padi (L.) (Hemiptera: Aphididae)), which commonly co-occurs with D. noxia. Because one type of resistance (mediated by the DO gene) is purported to be induced rather than constitutive, we also determined the degree to which the suitability of resistant and susceptible plants is modified by previous infestation. Although resistant lines were derived from five different sources, we did not detect any differences in the population growth of R. padi on resistant and near-isogenic susceptible plants. The performance of D. noxia was reduced as expected on resistant, plants but was unaffected by whether or not plants had been previously infested. In contrast, the performance of R. Padi was reduced on plants that had been previously infested but was unaffected by whether plants were resistant or susceptible to D. noxia. Because neither species' performance was especially low on previously infested, resistant plants, there was no evidence that prior aphid feeding induces a higher level of resistance.