Females of the weevil Zabrotes subfasciatus manipulate the size and number of eggs according to the host seed availability

Abstract

. In some insects, the finding of oviposition substrate triggers the uptake into oocytes of yolk proteins that are stored in the fat body during post‐embryonic development. The main host of the bean weevil Zabrotes subfasciatus (Coleoptera; Chrysomelidae; Bruchinae; Amblycerini), in which larval resources are the sole source for future egg maturation, is Phaseolus vulgaris. Despite not feeding as adults, females of this species are able to lay eggs after encountering host seeds but it is not known how females react to changes in the availability of bean seeds. In the present study, the behaviour of Z. subfasciatus facing two very different environments for oviposition is investigated, as well as how this influences offspring fitness. The results obtained show that females of Z. subfasciatus react to variations in the availability of seeds belonging to the same host species by adjusting egg size and number. Females on low bean seed density lay larger and fewer eggs than those on high bean seed density, demonstrating a trade‐off between these reproductive traits. Moreover, females can adjust egg size to changing levels of host availability during the first 4 days of their oviposition period. Although no difference in offspring weight is found, those from small eggs (low competition environment) result in larger adults. No response to selection on these traits after rearing beetles on the same host for 40 generations is observed. This unresponsiveness may indicate that beetle populations behave according to their reaction norm that already allows rapid adaptation to a varying amount of host‐seed availability and better exploitation of the environments of this widespread stored‐seed pest.

Intraspecific competition in Zabrotes subfasciatus: Physiological and behavioral adaptations to different amounts of host

“…The selection of oviposition sites occurs during the inspection period (Teixeira et al , 2009), when the female walks over the available seeds. The inspection period probably increases when the number of eggs per seed also increases (Chung et al , 1995).…”

“…Three artificial environments (treatments) were established by maintaining the number of Z. subfasciatus couples and varying the amount of available host seeds, thus promoting three levels of competition: (i) high competition (HC) with six couples and 1.35 g of grain; (ii) intermediate competition (IC) with six couples in 6 g of grain; and (iii) low competition (LC) with six couples in 36 g of grain. The LC level was previously defined as the presence of 15% of grains without eggs and most of the others (75%) with one or two eggs, and was used as reference for the other two treatments (HC and IC; see Teixeira et al , 2009). At every new generation, recently emerged individuals from a treatment (0–24 h) were put in the same conditions and this was repeated for 41 generations.…”

“…In a population with limited resources, genotypes tend to be constantly selected to recognize food more quickly, improve nutrient utilization and increase metabolic rates, thus leading to increased efficiency in resources usage (Pianka, 2000). However, the acquisition of positive adaptive traits, such as the use of a new host, is often associated with the acquisition of negative characteristics (trade‐off), like slower development, lower level of reproduction, reduced mobility and lower resistance to environmental changes (Remmert, 1982; Teixeira et al , 2009).…”

“…Adaptation to environmental alterations, such as an increased competition for food, depends on the development of a series of physiological and/or behavioral changes (Wassermann & Futuyma, 1981; Zucoloto, 1993; Gould, 2002; Teixeira & Zucoloto, 2003). These changes may be due to phenotypic plasticity, that is, the property of a given genotype to produce different phenotypes in response to distinct environmental conditions (Stearns, 1992; Pigliucci, 2002; West‐Eberhard, 2003; Teixeira et al , 2008, 2009). A genotype is non‐plastic if it shows the same phenotype in all environments (Pigliucci, 2001).…”

“…Environmental conditions can have deep impacts on the fitness and behavior of the adult insect, as seen when the larval density of a speckled Pararge aegeria L. population (Lepidoptera; Nymphalidae) was manipulated in order to have many individuals and low food availability, resulting in a drastic reduction in the population size, demonstrating the effects of intraspecific competition on the growth and development of this species (Gibbs et al , 2004). The bean weevil Zabrotes subfasciatus Boheman (1883) (Coleoptera; Chrysomelidae; Bruchinae; Amblycerini) can also ensure the survival of its offspring by ovipositing bigger, yet fewer, eggs, amid limited host availability (Teixeira et al , 2009).…”