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A herbivorous mite down-regulates plant defence and produces web to exclude competitors - PubMed

A herbivorous mite down-regulates plant defence and produces web to exclude competitors

Renato A Sarmento et al. PLoS One. 2011.

Abstract

Herbivores may interact with each other through resource competition, but also through their impact on plant defence. We recently found that the spider mite Tetranychus evansi down-regulates plant defences in tomato plants, resulting in higher rates of oviposition and population growth on previously attacked than on unattacked leaves. The danger of such down-regulation is that attacked plants could become a more profitable resource for heterospecific competitors, such as the two-spotted spider mite Tetranychus urticae. Indeed, T. urticae had an almost 2-fold higher rate of oviposition on leaf discs on which T. evansi had fed previously. In contrast, induction of direct plant defences by T. urticae resulted in decreased oviposition by T. evansi. Hence, both herbivores affect each other through induced plant responses. However, when populations of T. evansi and T. urticae competed on the same plants, populations of the latter invariably went extinct, whereas T. evansi was not significantly affected by the presence of its competitor. This suggests that T. evansi can somehow prevent its competitor from benefiting from the down-regulated plant defence, perhaps by covering it with a profuse web. Indeed, we found that T. urticae had difficulties reaching the leaf surface to feed when the leaf was covered with web produced by T. evansi. Furthermore, T. evansi produced more web when exposed to damage or other cues associated with T. urticae. We suggest that the silken web produced by T. evansi serves to prevent competitors from profiting from down-regulated plant defences.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Down-regulation of plant defence by T. evansi.

a Mean (+s.e.m.) oviposition rate of T. urticae on tomato leaf discs with damage and web of T. evansi (dam.+web), on leaf discs with damage by T. evansi, but without web (dam.), and on clean leaf discs (clean). b Mean (+s.e.m.) oviposition rate of T. evansi on tomato leaf discs with damage and web of T. urticae, on leaf discs with damage but without web of T. urticae, and on clean leaf discs. Different letters denote significant differences among treatments (lme with log likelihood ratio test). Experiments of panels a and b were not carried out at the same time for logistical reasons. Treatments should therefore be compared to the control carried out at the same time, presented in the same panel. Numbers inside the bars give number of replicates.

Figure 2
Figure 2. Population dynamics of T. evansi and T. urticae on tomato plants.

a Average numbers (± s.e.m., N = 4 plants) of T. urticae when alone on a plant (closed circles) or when together with T. evansi (open circles). b Average numbers of T. evansi when alone (closed circles) or when together with T. urticae (open cicrles). At the end of the experiment, the plants were overexploited and mites would start dispersing. Data from the two single-species experiments were presented in .

Figure 3
Figure 3. Web of T. evansi hinders competitors.

The average proportion (+s.e.m., N = 10) of adult female T. urticae on the leaf surface after 1.5 h (left-hand two bars), after 17.5 h (middle two bars) and the average proportion of mites feeding after 17.5 h (right-hand two bars), on tomato leaflets that were infested with 100 adult female T. evansi. The web produced by T. evansi was removed from half of the leaves (web removed), and was left intact on the other half of the leaflets. Asterisks above the bars indicate significant difference between the two treatments: *: p<0.05; **: p<0.005.

Figure 4
Figure 4. Local cues of competitors induce web production in T. evansi.

a Mean (+s.e.m., N = 7) oviposition rate of T. evansi on tomato leaf discs that were previously infested with conspecific mites (T.e. dam.), with T. urticae (T.u. dam.), and on clean leaves (clean). b Mean (+s.e.m.) relative web density of T. evansi on tomato leaf discs that were previously infested with conspecifics (T.e. dam.), with T. urticae (T.u. dam.), and on clean leaves (clean). Different letters denote significant differences among treatments (lme with log likelihood ratio test). Numbers inside the bars are number of replicates.

Figure 5
Figure 5. Distant cues of competitors induce web production in T. evansi.

Effect of cues from clean leaves or leaves infested by T. urticae or T. evansi on the production of eggs and web by T. evansi. a Effect of cues from leaves with T. urticae on oviposition of T. evansi. b Effect of cues from leaves with T. evansi on oviposition of T. evansi. c Effect of cues from leaves infested with T. urticae on web production of T. evansi. d Effect of cues from leaves infested with T. evansi on web production of T. evansi. * denotes a significant difference between treatments (lme with log likelihood ratio test, p = 0.03); ns means no significant differences (p>0.05). For logistical reasons, experiments of panels a plus b and c plus d were not carried out at the same time. Therefore, treatments can only be compared to their controls within the same experiment. Numbers inside bars give the number of replicates.

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