Effects of kinship or familiarity? Small thrips larvae experience lower predation risk only in groups of mixed-size siblings

Abstract

In many species of insects, larvae are distributed in an aggregated fashion. As they may differ in size and size matters to predation risk, small larvae may be less likely to fall prey to predators when near large and therefore better-defended larvae. We hypothesize that the small larvae may profit even more when these large larvae are siblings. We tested this hypothesis on kinship-dependent survival in groups of larvae of the Western flower thrips (Frankliniella occidentalis) exposed to a predatory mite (Iphiseius degenerans). Our experiments showed that small larvae in sibling groups survive significantly better than in non-sibling groups, but only when such groups consisted of a mixture of small and large larvae. To test whether the survival effect we found is due to familiarity of thrips larvae growing up together (i.e. on one leaf), we also measured survival in sibling groups of larvae grown up on different leaves and in non-sibling groups of larvae grown up on the same leaf. These experiments showed an increased survival of small thrips larvae only in groups of sibling larvae from the same leaf. Non-sibling larvae did not show an increased survival when they come from the same leaf. Our results indicated that the increased survival in sibling groups was only partly due to the familiarity effect we tested. Growing up together did not return the same survival effect for non-siblings as it did for siblings. We conclude that growing up together is a necessary but not sufficient condition for discrimination in thrips larvae.

Keywords: Frankliniella occidentalis; Iphiseius degenerans; Kin discrimination; Non-social insects; Predation risk; Size structure.

Fig. 1

Survival (left panels) and mortality rate (right panels) of thrips larvae in the presence of the predatory mite I. degenerans. On the x-axes is the time in days. On the y-axis of the left panels is the average number of surviving thrips larvae during 4.3 days in sibling groups (dark green filled boxes) or non-sibling groups (light green open boxes) that were composed of a five first-instar and five second-instar larvae (N = 20 for sibling groups, 19 for non-sibling groups), b ten first-instar larvae (N = 31 for sibling groups, 35 for non-sibling groups) and c ten second-instar larvae (N = 19 for sibling groups, 19 for non-sibling groups). To facilitate comparison of the survival data, the right panels show the corresponding average mortality rates (day⁻¹) calculated from the survival measurements. Note the difference in mortality rate at the start of the experiment in panel a. Error bars show standard errors

Fig. 2

Survival of thrips larvae in mixed-size groups of siblings or non-siblings during 4.3 days. The data are the same as in Fig. 1a, but displayed separately for first-instar larvae and second-instar larvae. On the x-axis is the time in days, on the y-axis the fraction of surviving individuals. First-instar larvae are presented with triangles and a solid line, second-instar larvae with circles and dotted line; sibling groups in dark green and non-sibling groups in light green

Fig. 3

Survival of thrips larvae in mixed-size groups of siblings or non-siblings during 4.3 days in absence of predation. On the x-axis is the time in days, on the y-axis the number of surviving individuals. Error bars show standard errors. N = 12 for both sibling groups (dark green filled boxes) and non-sibling groups (light green open boxes). Error bars show standard errors

Fig. 4

Survival of sibling thrips larvae in mixed-size groups after 6 h of exposure to predation. On the x-axis are the treatments, SSL refers to sibling same leaf, SDL refers to sibling different leaf. On the y-axis is the number of surviving individuals. The number of replicates is 26 for SSL, 27 for SDL. Error bars show standard errors. a Survival of the first- and second-instar larvae together. b Survival of the first-instar larvae and c survival of the second-instar larvae

Fig. 5

Survival of non-sibling thrips larvae in mixed-size groups after 6 h of exposure to predation. On the x-axis are the treatments, NSSL refers to non-sibling same leaf, NSDL refers to non-sibling different leaf. On the y-axis is the number of surviving individuals. The number of replicates is 25 for NSSL, 26 for NSDL. Error bars show standard errors. a Survival of the first- and second-instar larvae together. b Survival of the first-instar larvae and c survival of the second-instar larvae