Seed suitability and oviposition behaviour of wild and selected populations of Zabrotes subfasciatus (Boheman) (Coleoptera: Bruchidae) on different hosts

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Abstract

The major host for Zabrotes subfasciatus (Boheman) (Coleoptera: Bruchidae) is the common bean (Phaseolus vulgaris L.). The seeds of this host, which have been developed into many commercial varieties, vary greatly in size and colours. These are sympatric to other host species of Leguminosae which apparently are not typically used as hosts, but with which Z. subfasciatus also has contact. The response of a wild population of beetles in terms of acceptability and host suitability was observed in five different varieties of the usual host (varieties Bolinha amarela, Branco, Carioquinha, Jalo, and Preto) and in three atypical host species (chickpea, soy, and lentil). The data were compared with those from selected populations, which spent seven generations in each host. The adaptation ability of the wild population to different hosts was observed and it was verified that both the oviposition behaviour and the host suitability were modified in a few generations in some of the beetle populations. The results also allowed us to discuss the influence of oviposition behaviour and larval performance on host range determination.

Introduction

In holometabolous insects, where the immatures have restricted movements, the oviposition behaviour of adults and larval development in the host are decisive factors in terms of host range (Wasserman and Futuyma, 1981; Janz and Nylin, 1997; Carrière, 1998). There are many factors that may influence the oviposition of insects on a specific host, such as morphology (Johnson and Kistler, 1987), nutritional quality, host abundance (Janz and Nylin, 1997; Barros and Zucoloto, 1999), places free of predators (Bernays and Graham, 1988) and competition (Siemens et al., 1991). Bruchid beetles are excellent subjects for empirical studies on the evolutionary and biological theories related to oviposition and to host use because they are easy to manipulate and the laboratory can simulate the grain storage environment (Fox and Tatar, 1994).

Wasserman and Futuyma (1981) established two basic components related to the use of a host by a species of Bruchidae (Callosobruchus maculatus (F.)). The first was the decision taken by the female whether or not to lay eggs on a particular seed. This is defined by the term acceptability, which describes the likelihood of a plant being accepted if it is encountered by an insect. This term was used in the present study rather than preference, because no host-choice experiments were performed (Singer, 1986). The second component is the ability of the larvae to develop inside this particular host. Host suitability is defined as survival, growth, and reproduction of an insect after feeding on a host (Singer, 1986). The bean (Phaseolus vulgaris L.) is the usual host of Zabrotes subfasciatus (Boheman). The adult female determines the quality and quantity of food for its immatures (Dendy and Credland, 1991), because larvae are restricted to the place where the female had laid the egg. When eggs hatch the larvae enter the seed where all development occurs (Southgate, 1979). The adult does not usually feed (Carvalho and Rosseto, 1968; Abate and Ampofo, 1996), which makes the choice of oviposition sites even more important to the biological success of the descendants. A female may have a variety of oviposition sites, i.e., in addition to different varieties of beans (which vary greatly in size and colour), she may choose other Leguminosae species that are also stored in buildings where Z. subfasciatus mainly occurs (Carvalho and Rosseto, 1968).

The objectives of the present study were to determine whether there are differences in host suitability for this bruchid and the acceptability of different varieties of their main host and of other species of potential hosts, and to observe the specificity of a wild population and the adaptive modifications that occurred in the population after several generations in other hosts. The experiments were conducted with a wild population of Z. subfasciatus and five commercial varieties of beans and three species of Leguminosae common to the region where the wild population was collected.

Section snippets

Materials and methods

The wild Z. subfasciatus used for the experiments were acquired from infested seed collected in houses and stores from the Ribeirão Preto region, SP, Brazil (21°05′–21°15′S and 47°50′–47°55′W). Five commercial varieties of the usual host, P. vulgaris, (Bolinha amarela, Branco, Carioquinha, Jalo, and Preto) and three species of atypical hosts including chickpea (Cicer arietinum L.), soy (Glycine max L.), and lentil (Lens culinaris Medik) were used. Several characteristics of these varieties and

Results

The characteristics of the hosts were different in many respects (Table 1). The acceptability was high for all varieties of the usual host (Table 2). The atypical hosts had lower levels of acceptability by the insects than the usual hosts. Chickpea was the atypical host on which the females of Z. subfasciatus showed highest oviposition rejection. When the insect population from chickpea was placed on bean the number of eggs was very high. This shows the influence of rejection of a host on the

Discussion

The present study confirmed that oviposition behaviour and host suitability can independently influence the use of a host. This could possibly lead to an increase in the acceptability of a host and to an improvement of the suitability of a particular host after several generations of insect development if a range of natural variation in the insect population is present.

The high number of eggs laid on all varieties of P. vulgaris, despite their variations in size and colour, shows that there

Acknowledgements

CAPES and CNPq supported this work.

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