Abstract
Functional response describes the number of prey or hosts attacked by a predator or parasitoid as a function of prey or host density. Using three different experimental designs, we found a linear functional response by two insect parasitoids (the pteromalid Pachycrepoideus vindemiae and the diapriid Trichopria drosophilae) to their hosts (the drosophilids Drosophila suzukii and D. melanogaster). A linear function response is considered unusual for insect parasitoids. The first design was a ‘fixed time within patch experiment’ where individual parasitoids were exposed to a range of host densities for 24 h; the second two designs were a ‘variable time functional response’ and a ‘selective functional response’ experiments where individual parasitoids were presented with a range of host patches and allowed to freely select and explore only one patch (variable time) or forage for 24 h (selective). In all experimental designs, the number of hosts parasitized increased linearly until reaching an upper limit. Under the laboratory conditions used, the functional response of P. vindemiae was limited by its egg supply and time (host handling time) whereas T. drosophilae was limited by time only. The linear functional response by both parasitoids likely resulted from a constant attack rate and an incremental foraging strategy where the parasitoids left a poor (low density) host patch or remained in a higher quality host patch when there was successful oviposition and adequate host density.
Highlights
Parasitoid functional response describes the number of hosts attacked by individual parasitoids as a function of host density [1], and has been both studied to assess the biological control potential of parasitoids and used as an essential component of parasitoid–host models [2,3,4]
We explored the behavioral mechanisms underlying the functional response of P. vindemiae and T. drosophilae using three different experimental designs: fixed time and within patch, variable time, and selective functional response
P. vindemiae appears to have depleted its egg supply after the 24 h exposure, but developed more eggs at 48 h, whereas T. drosophilae held more than 25 mature eggs at both time periods (Fig 3B), suggesting that P. vindemiae likely had suffered from egg supply while T. drosophilae was not yet egg-limited or time-limited when the functional response curve approached to an asymptote at the high host densities for a fixed exposure time
Summary
Parasitoid functional response describes the number of hosts attacked by individual parasitoids as a function of host density [1], and has been both studied to assess the biological control potential of parasitoids and used as an essential component of parasitoid–host models [2,3,4]. Holling [1] proposed three functional response curves in response to increasing host density over a fixed time-period: a linear increase (type I) where the attack rate is constant (density independence), a negative acceleration (type II) where the attack rate gradually declines (inverse density dependence), and a sigmoidal curve (type III) where the attack rate first. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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