Gall-inducing insects display intimate interactions with their host plants, usually described as parasitic relationships; the galls seem to favor the galler alone. We report on a case in which the presence of the galls induced by Leptocybe invasa Fisher & LaSalle (Hymenoptera; Eulophidae) benefit its host plant, the river red gum Eucalyptus camaldulensis Dehnh. Field observations showed that E. camaldulensis plants infected by this gall wasp were less susceptible to cold injury than neighboring conspecific plants without galls. In the laboratory, frost resistance was compared between galled and non-galled plants which were both divided into two subgroups: cold-acclimated plants and plants that were non-acclimated. Galled plants displayed higher frost resistance than the non-galled ones, and the differences were higher in non-acclimated plants compared with acclimated ones. Physiological changes in host plant were determined by chemical analyses of chlorophylls, proteins, soluble sugars and anthocyanin contents. The results showed higher values of all physiological parameters in the galled plants, supporting the hypothesis that the presence of the gall wasp induces physiological changes on the plant foliage, which may in turn increase plant defense mechanisms against cold. Therefore, the toll of galling by the herbivore may pay off by the host plant acquiring increased frost resistance. This work provides evidence for physiological changes induced by a herbivore which might have a positive indirect effect on the host plant, promoting frost resistance such as cold acclimation.
• The defense hypothesis of Ni hyperaccumulation was tested in Alyssum pintodasilvae.• We compared the effects of A. pintodasilvae and A. simplex on Tribolium castaneum.• No-choice and choice tests were performed using diet disks amended with leaves.• Both high-Ni and low-Ni plants caused significant antifeedant effects on Tribolium.a b s t r a c t a r t i c l e i n f o The defense hypothesis is commonly used to explain the adaptive role of metal hyperaccumulation. We tested this hypothesis using two Brassicaceae congeneric species: Alyssum pintodasilvae, a Ni hyperaccumulator, and the non-accumulator Alyssum simplex both growing on serpentine soils in Portugal. Artificial diet disks amended with powdered leaves from each plant species were used to compare the performance (mortality, biomass change) and feeding behavior of Tribolium castaneum in no-choice and choice tests. The performance of T. castaneum was not affected at several concentrations of A. pintodasilvae or A. simplex in no-choice tests. However, the consumption of plant-amended disks was significantly lower than that of control disks, irrespectively of the species fed. Accordingly, when insects were given an alternative food choice, disks of both plant species were significantly less consumed than control disks. Moreover, insects did not discriminate between disks in the combination "A. pintodasilvae + A. simplex". Contrary to our expectations, these results suggest that both plant species have equally effective defenses against herbivory. While Ni is believed to be part of the deterrence mechanism in the hyperaccumulator A. pintodasilvae, it seems likely that organic compounds, possibly glucosinolates, play an important role in the defense of A. simplex or in both species.
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