Caging overrides effects of density in laboratory-based physiological estimates of reproductivity in Listronotus bonariensis (Coleoptera: Curculionidae)

Abstract

This contribution examines the potential for error when the results from laboratory-based experiments are extrapolated to inform field ecological analyses. The effects on the reproductivity of different population densities of Argentine stem weevil Listronotus bonariensis (Kuschel) were studied using caged populations of immature (pre-reproductive) and mature (reproductive) adults. Three densities of weevils equivalent to 125/m² (low), 375/m² (medium) and 1125/m² (high) from field-collected pre- and reproductive populations were confined in plastic containers for either 11 or 21 days with ad libitum food and oviposition-site resources. The impact of the medium and high densities on weevil reproductive physiology was assessed by dissection at 11 and 21 days. Data were collected on sexual maturity, wing muscle development, and the presence of eggs and β-carotene crystals. Only very weak treatment effect responses were observed. A significantly higher percentage of immature females showed oöcyte resorption at high densities at 11 days, but by 21 days this had disappeared. Males had significantly less developed testes at high densities at 11 days, but again by 21 days this effect was not significant. Mature females had significantly larger wing muscles at high density after 11 days of caging; however, by 21 days this effect had gone. For both immature and mature females, there was no effect of density on the relationship between eggs per female and wing muscle index. The low density treatment was not included in this analysis due to low weevil numbers surviving at the end of the experiment. It is concluded that caging itself had a far greater effect on the physiology of the weevils than any imposed population density treatments, which is contrary to the findings of field population-dynamics analyses. Care should be taken in extrapolating laboratory trials involving this genus, and indeed it may occur more widely, to explain field situations.