The influence of the diet of the Argentine stem weevil, Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae) on the fitness of its parasitoid, Microctonus hyperodae loan (Hymenoptera: Braconidae): prospects for 'indirect' conservation biocontrol

Abstract

The Argentine stem weevil, Listronotus bonariensis (Kuschel) (Coleoptera: Curculionidae), is the most important pest species in New Zealand, costing the pastoral industry an estimated NZ$78-251 m per year (Prestidge et al., 1991). The parasitoid wasp Microctonus hyperodae Loan (Hymenoptera: Braconidae) was released in New Zealand in 1991 as a potential biological control agent of the weevil (Goldson et al., 1994). As M. hyperodae is a pro-ovigenic species, its lifetime complement of eggs is set at adult emergence (Phillips, 1998). Adult wasp fitness is likely to be affected by host nutrition, as resources carried over from immature stages to the adult stage of parasitoids can vary with the size and nutritional quality of the host (Jervis et al., 2001). The fecundity of Argentine stem weevil can increase after the addition of pollen to its diet (Evans. and Barratt, 1995). Conversely, a decrease in gonad development (i.e., vitellarium development and percentage of females with oocyte resorption) can occur after consumption of ryegrass containing endophyte (Barker and Addison, 1996). This research investigates tri-trophic-Ievel interactions on parasitoid fitness (i.e., egg load and body size) by assessing field-collected adult weevils, and by providing bee-collected pollen, buckwheat (Fagopyrum esculentum Moench cv. Katowase) pollen, ryegrass-only (Lolium multiflorum L. cv. Tama), or ryegrass (Lolium multiflorum Lambrechtsen cv. Aries) containing endophyte (Neotyphodium lolii (Latch, Christensen and Samuels) Glenn, Bacon and Hanlin in the host diet in the laboratory.

A survey was conducted in the Lincoln area to what extent pollen was part of the diet of field collected Argentine stem weevils. The frequency and types of pollen grains consumed were analysed. Also, gut fullness (in terms of ryegrass particle content), parasitoid larval presence, weevil gender and weevil gonad development were also assessed. Argentine stem weevil adults fed on pollen in the field, although a low proportion of them contained pollen in their gut. Gonad development was positively correlated with the extent of pollen consumption (i.e., number of individuals with sexually mature gonads).

A cage experiment was conducted in the laboratory. Potentially parasitised adult Argentine stem weevils were collected from ryegrass on roadsides in the Lincoln area during 2004. They were randomly allocated to four treatments: ryegrass-only (control), ryegrass plus bee collected pollen, ryegrass plus buckwheat pollen, and ryegrass with endophyte, each replicated four times. Water was provided in every treatment. The parasitoids that emerged were assessed for size (i.e., hind leg tibial length) and potential fecundity (i.e., egg load) (see Phillips and Baird, 2001). Despite the fact that the addition of bee-collected or buckwheat pollen to the host diet had no significant effect on the number of adult parasitoids emerged, their body size or egg load (P > 0.05), some factors that could have affected a potentially positive correlation between parasitoid fitness and pollen consumption are discussed. Similarly, the addition of ryegrass with endophyte had no significant effect on the number of adult parasitoids emerged, their body size or egg load (P > 0.05). This leads to the suggestion that biological control (i.e., M. hyperodae) and plant resistance (i.e., endophyte) strategies can work in a complementary way in integrated pest management programmes.

The concept of benefiting the third trophic level with 'resource subsidies' made available selectively to the second level has the potential to lead to the suggestion of a new mechanism for enhancing parasitoid fitness, that is 'indirect' conservation biocontrol.