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Abstract. In many insect species, the size and number of eggs decrease with maternal age. Thus, both the size and number of eggs must be considered to know the exact cost of reproduction with maternal age. The resource depletion hypothesis was examined in the bruchid beetle Callosobruchus chinensis. The hypothesis explains why the egg size decreases with maternal age based on the decline of the female's reproductive capacity. A decrease was found in reproductive effort ( egg size  the number of eggs) and the fitness component of offspring with maternal age. The effects of the female's nutritional status on the relationship between maternal age and the reproductive effort of females with and without food and water were also examined. The results indicate that the decrease in size and number of eggs with maternal age can be explained by the resource depletion hypothesis in C. chinensis.

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Summary1. Transgenerational effects, genetic or non-genetic, affect population dynamics and the evolution of life-history traits. Besides genetic components, the size of gametes (eggs and seeds), simultaneously a parental and progeny character, can mediate environmental condition experienced by a parent. In both animals and arthropods, mothers are known to reduce their egg mass depending on their malcondition. 2. Progeny may also modify their life history traits to increase their own fitness when constrained by maternal investment, which may eventually nullify transgenerational effects on population dynamics and evolution. Such fitness modification by the progeny under new environmental conditions requires phenotypic plasticity interacting with egg mass. We hypothesize that different selective environments should produce inter-population genetic diversification of the response to maternal investment on each egg, which would be detected as a paternal genotype · environment · previous (i.e. maternal) environment (G · E · preE) interaction in progeny fitness. 3. To evaluate the contribution of maternal non-genetic resource and the genetic component separately, we used an inbred-isofemale-line approach to eliminate the influence of the genetic correlation between egg mass and other life history traits, in the adzuki bean beetle, Callosobruchus chinensis. The females were reared at either high or low densities to generate variability in egg resources. To test the additive or interactive effect of genotype, non-genetic egg resources, and maternal environment on the life history traits of the progeny, they were crossed with males from laboratory and wild strains that had been subjected to different levels of population density. 4. The G · E · preE interaction effect was detected on the correlation structure between egg mass and development time: In the offspring of mothers reared at low density, the negative correlation between egg mass and development time was higher with lab strain fathers, whereas in the offspring of mothers reared at high density, the negative correlation was higher with the wild strain fathers. 5. Our results indicate a genetic difference in the response of development time but not of adult mass to environmental variation in egg mass. Such density-dependent enhancement of maternal effects may destabilize population dynamics and accelerate evolution.

Life‐history theory predicts that females who experienced stressful conditions, such as larval competition or malnutrition, should increase their investment in individual offspring to increase offspring fitness (the adaptive parental hypothesis). In contrast, it has been shown that when females were reared under stressful conditions, they become smaller, which consequently decreases egg size (the parental stress hypothesis). To test whether females adjust their egg volume depending on larval competition, independent of maternal body mass constraint, we used a pest species of stored adzuki beans, Callosobruchus chinensis (L.) (Coleoptera: Chrysomelidae: Bruchinae). The eggs of females reared with competitors were smaller than those of females reared alone, supporting the parental stress hypothesis; however, correcting for female body size, females reared with competitors produced larger eggs than those reared in the absence of competition, supporting the adaptive parental hypothesis, as predicted. The phenotypic plasticity in females' investment in each offspring in stressful environments counteracts the constraint of body size on egg size.