Predation risk estimated on live and artificial insect prey follows different patterns

Models mimicking prey organisms are increasingly used in ecological studies including testing fundamental ecological and evolutionary theories. The general consensus is that predation risk estimated on artificial models may not quantitatively correspond to predation pressure on live prey, but it still can be used in various comparisons. We tested whether the use of live and artificial prey reveals the same patterns of variation in predation risk. We exposed live prey (blowfly larvae and puparia) and plasticine models of blowfly puparia in two boreal forest sites, both openly and in ant- and bird-exclusion treatments, and we quantified attacks by both avian and invertebrate predators. Bird attack rates were always higher on live puparia than on their plasticine models, but the magnitude of this difference declined from 8.4-fold in early summer to 2-fold in mid- and late summer. We attribute these changes to different responses to prey by experienced adult birds that dominate the bird communities in early summer versus explorative juvenile birds that are abundant later in the season. Invertebrate daily predation rates on maggots decreased from 56% in early summer to 28% in late summer, but invertebrate attacks on plasticine models showed no seasonal changes. Overall, invertebrate predation on maggots was 67-fold greater than their predation on models. Observations showed that wood ants did not attack plasticine models and did not leave on them any damage marks. Estimates based on artificial prey indicate a much greater role of bird predation than invertebrate predation, while estimates based on live prey suggest the opposite pattern. Thus, using live and artificial prey may lead to different conclusions about relative importance of different predator groups in a locality. Moreover, for both avian and invertebrate predators, predation risk based on artificial and live prey shows different seasonal changes and may potentially demonstrate different spatial patterns.Funding provided by: Academy of FinlandCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100002341Award Number: 311929Funding provided by: Academy of FinlandCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100002341Award Number: 316182In our study, we used live larvae (maggots hereafter) and puparia of the blowfly Calliphora sp. (Diptera: Calliphoridae) and plasticine models with similar appearance to fly puparia in size, colour and shape. Maggots, obtained in a local fishing bait shop, were 10–13 mm long. They were divided into two equal parts: one part was placed in a refrigerator (at 4°C) to prevent pupation and the other part was kept at room temperature (22°C) and allowed to pupate. We mixed red and black plasticine (advertised as non-toxic and odourless; Chemical plant "Luch", Yaroslavl, Russia) in the proportion 4:1 to obtain a colour (brown) similar to that of puparia, and we moulded models from this plasticine. Three individual models were attached in a line along a stick (about 10 cm long, cut from thin birch branches) using universal glue (Yleisliima; Biltema Suomi Oy, Helsinki, Finland). In the same fashion we glued three puparia. The live and plasticine puparia set in this way mimicked some caterpillar-like prey. The data were obtained from three different experiments. Experiment 1 was designed to compare bird and invertebrate predation on three different prey items. For this experiment, at each site, we selected 10 pairs (blocks) of young (2–3 m tall) downy birches (Betula pubescens). The distance between the paired trees was 0.5–2 m, and the distance between blocks was about 20 m. One tree in each block was isolated from non-flying invertebrate predators (mostly ants) by applying a ring of non-drying glue (Sticky-Trap, Vilofarm A/S, Hobro, Denmark) at the base of the trunk (ant exclusion hereafter). All ants and other invertebrates were removed by shaking these isolated trees and then manually. We ensured that the branches and foliage of any ant-exclusion plants did not touch other trees to prevent any migration of non-flying invertebrate predators (with possible exception of spiders) from neighbouring trees. The second tree in each block remained accessible to ants and other predators (control hereafter). On each of 20 trees, we placed three kinds of prey items: an open transparent plastic vial (40 mm height and 30 mm diameter) containing three maggots, one stick with three puparia and one stick with three models. The vial was attached with tape to the tree trunk 80–100 cm above the ground. Each vial also contained a small stick to ease the movement of predatory invertebrates (we had previously verified that the maggots were incapable of climbing on the stick). Sticks with puparia and models were attached with small pieces of wire to birch branches at 100–150 cm above the ground at the outer part of the crown, so that the distance between them was as far as possible. Experiment 1 was repeated six times in Kustavi and seven times in Turku using the same trees from 28 May to 11 September 2021. At 24 h from the start of the experiment, we recorded (i) the number of maggots that had disappeared from each vial, (ii) the number of dead maggots in each vial; (iii) the number of damaged puparia; and (iv) the presence of attack marks by both invertebrates and birds on each of the three plasticine models. At 4–7 days from the start of the experiment, we repeated records on live puparia and plasticine models. Experiment 2 was conducted to explore whether birds attacked the maggots exposed in vials. At the beginning of June 2021, at both study sites, we selected 20 young birches and attached a vial with five maggots to the trunk, as described above. To exclude bird predation, we covered the openings of the vials on 10 of these birches with bird netting. The number of maggots that disappeared from vials was recorded at 2 days from the start of the experiment. Experiment 3 was conducted to compare invertebrate attacks on maggots and puparia. In early June of 2021, we placed two vials (as in experiment 1) on each of 20 birch trees in each of two sites. Half of the trees were isolated from ants by glue rings, as described above. We placed three maggots in one of these vials and three puparia into the other. The number of prey individuals that disappeared from vials was recorded at 4 days from the start of the experiment