Abstract
Changes in plants following insect attack are referred to as induced responses. These responses are widely viewed as a form of defence against further insect attack. In the current study we explore whether it is possible to make generalizations about induced plant responses given the unpredictability and variability observed in insect-plant interactions. Experiments were conducted to test for consistency in the responses of two congeneric thrips, Frankliniella schultzei Trybom and Frankliniella occidentalis Pergrande (Thysanoptera: Thripidae) to cotton seedlings (Gossypium hirsutum Linneaus (Malvales: Malvaceae)) damaged by various insect herbivores. In dual-choice experiments that compared intact and damaged cotton seedlings, F. schultzei was attracted to seedlings damaged by Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), Tetranychus urticae (Koch) (Trombidiforms: Tetranychidae), Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae), F. schultzei and F. occidentalis but not to mechanically damaged seedlings. In similar tests, F. occidentalis was attracted to undamaged cotton seedlings when simultaneously exposed to seedlings damaged by H. armigera, T. molitor or F. occidentalis. However, when exposed to F. schultzei or T. urticae damaged plants, F. occidentalis was more attracted towards damaged plants. A quantitative relationship was also apparent, F. schultzei showed increased attraction to damaged seedlings as the density of T. urticae or F. schultzei increased. In contrast, although F. occidentalis demonstrated increased attraction to plants damaged by higher densities of T. urticae, there was a negative relationship between attraction and the density of damaging conspecifics. Both species showed greater attraction to T. urticae damaged seedlings than to seedlings damaged by conspecifics. Results demonstrate that the responses of both species of thrips were context dependent, making generalizations difficult to formulate.
Highlights
Insect herbivores are diverse and feed on plants in various ways
Of the studies that investigated the effect of plant induction on oviposition responses (n = 30), 33% recorded an increase in oviposition on induced plants, 63% recorded a decrease and 33% recorded that plant induction had no effect (Table 1)
F. schultzei was more attracted to herbivore-damaged plants compared to undamaged plants, regardless of whether herbivores were in situ or if they had been removed (H. armigera in situ: x2 = 38.4, d.f. = 1, P,0.0001; H. armigera removed: x2 = 40.6, d.f. = 1, P,0.0001; root damage: x2 = 26.6, d.f. = 1, P,0.0001; thrips (F. schultzei ) in situ: x2 = 22.4, d.f. = 1, P,0.0001; thrips (F. schultzei ) removed: x2 = 39.8, d.f. = 1, P,0.0001; thrips (F. occidentalis) removed: x2 = 32.2, d.f. = 1, P,0.0001; mites in situ: x2 = 34.8, d.f. = 1, P,0.0001; mites removed: x2 = 19.2, d.f. = 1, P,0.0001; Figure 1)
Summary
Insect herbivores are diverse and feed on plants in various ways. Plants respond biochemically to herbivore, and pathogen, attack [1,2] and these induced responses are widely considered to constitute a form of defence [3,4]. Herbivore-induced plant interactions become more intricate when herbivore natural enemies are considered. Results from specific studies have frequently been used to support functional interpretation of induced responses including their ecological impact, their effect on the fitness of the impacted herbivores and even possible coevolutionary relationships between plants and the natural enemies of their herbivores [9]. It begs the question: under what circumstances can we make generalizations about induced plant responses to insect herbivory? It begs the question: under what circumstances can we make generalizations about induced plant responses to insect herbivory? put, can we extrapolate from specific studies to draw broad scale conclusions about induced plant responses, or are conclusions from specific studies only valid to the particular systems studied?
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