Adaptive learning in arthropods: spider mites learn to distinguish food quality

Learning in herbivorous insects: dispersing aphids spend less time evaluating familiar than novel non-host plant species

For many organisms, dispersal may be a high-risk activity, and dispersers are likely to have behavioral, physiological, or other adaptations that increase the probability they will successfully settle in new habitat. Dispersing aphids, for example, are small-bodied, relatively weak flyers that must navigate through a complex landscape where non-host species may be much more common than suitable hosts are. While previous research has focused on how dispersing aphids locate and evaluate host species, little is known about how they interact with the non-host species they encounter while host searching. Here, I report on an experiment to test the hypothesis that dispersers of Aphis fabae spend less time evaluating non-host species with which they have had prior experience than novel non-host species. Aphids consistently spent less time in contact with familiar non-host species than novel non-host species, but the magnitude of this effect varied for different non-host species. Aphids that had previously encountered rose spent less time interacting with rose than with raspberry or goldenrod, and aphids that had previously encountered raspberry spent less time interacting with raspberry than with goldenrod. Aphids that had previously encountered goldenrod showed a less pronounced and statistically non-significant reduction in time spent interacting with goldenrod relative to either raspberry or rose. The ability to recognize previously encountered non-hosts may allow aphids to navigate more efficiently through an environment in which they face many more non-hosts than hosts, and therefore increase the probability that a disperser will ultimately locate and settle on an appropriate host plant.

Induced Preference Improves Offspring Fitness in a Phytopathogen Vector

In the Asian citrus psyllid (Diaphorina citri Kuwayama), learning facilitates host recognition and mate preference; however, it is unclear whether induced female oviposition preference occurs in this species. We investigated the influence of natal host experience on adult oviposition preference when reared on either 'Valencia' orange (Citrus x sinesis) or orange jasmine (Murraya paniculata). Psyllids reared on 'Valencia' orange preferred 'Valencia' orange as an oviposition host compared with orange jasmine, whereas there was no difference in oviposition between the two hosts in orange jasmine reared psyllids. Nymphs transferred from 'Valencia' orange to orange jasmine were smaller in adult size and required more time for development. These findings were reversed in orange jasmine reared psyllids, which increased in size and displayed shorter development times when transferred from orange jasmine to 'Valencia' orange. However, mortality increased in nymphs transferred to the non-natal host species in both treatment groups compared with nymphs transferred to the same host. These results indicate an association between host plant preference and performance in this species. Maternal host experience appeared to influence the oviposition preference in this species. Juvenile psyllid performance appeared negatively affected by orange jasmine plants such that fitness was reduced, suggesting benefits for maternal host fidelity in those insects not acclimated to feeding on orange jasmine. Induced oviposition preference may provide an important mechanism of adaptive plasticity in D. citri reproductive strategies, allowing females to discriminate among potential host species in favor of those to which her offspring are best adapted.

Age-dependent male mating tactics in a spider mite-A life-history perspective

Males often fight with rival males for access to females. However, some males display nonfighting tactics such as sneaking, satellite behavior, or female mimicking. When these mating tactics comprise a conditional strategy, they are often thought to be explained by resource holding potential (RHP), that is, nonfighting tactics are displayed by less competitive males who are more likely to lose a fight. The alternative mating tactics, however, can also be explained by life‐history theory, which predicts that young males avoid fighting, regardless of their RHP, if it pays off to wait for future reproduction. Here, we test whether the sneaking tactic displayed by young males of the two‐spotted spider mite can be explained by life‐history theory. We tested whether young sneaker males survive longer than young fighter males after a bout of mild or strong competition with old fighter males. We also investigated whether old males have a more protective outer skin—a possible proxy for RHP—by measuring cuticle hardness and elasticity using nanoindentation. We found that young sneaker males survived longer than young fighter males after mild male competition. This difference was not found after strong male competition, which suggests that induction of sneaking tactic is affected by male density. Hardness and elasticity of the skin did not vary with male age. Given that earlier work could also not detect morphometric differences between fighter and sneaker males, we conclude that there is no apparent increase in RHP with age in the mite and age‐dependent male mating tactics in the mite can be explained only by life‐history theory. Because it is likely that fighting incurs a survival cost, age‐dependent alternative mating tactics may be explained by life‐history theory in many species when reproduction of old males is a significant factor in fitness.

Colonisation rate and adaptive foraging control the emergence of trophic cascades

Ecological communities are assembled and sustained by colonisation. At the same time, predators make foraging decisions based on the local availabilities of potential resources, which reflects colonisation. We combined field and laboratory experiments with mathematical models to demonstrate that a feedback between these two processes determines emergent patterns in community structure. Namely, our results show that prey colonisation rate determines the strength of trophic cascades - a feature of virtually all ecosystems - by prompting behavioural shifts in adaptively foraging omnivorous fish predators. Communities experiencing higher colonisation rates were characterised by higher invertebrate prey and lower producer biomasses. Consequently, fish functioned as predators when colonisation rate was high, but as herbivores when colonisation rate was low. Human land use is changing habitat connectivity worldwide. A deeper quantitative understanding of how spatial processes modify individual behaviour, and how this scales to the community level, will be required to predict ecosystem responses to these changes.

Intraspecific variation in a generalist herbivore accounts for differential induction and impact of host plant defences

Plants and herbivores are thought to be engaged in a coevolutionary arms race: rising frequencies of plants with anti-herbivore defences exert pressure on herbivores to resist or circumvent these defences and vice versa. Owing to its frequency-dependent character, the arms race hypothesis predicts that herbivores exhibit genetic variation for traits that determine how they deal with the defences of a given host plant phenotype. Here, we show the existence of distinct variation within a single herbivore species, the spider mite Tetranychus urticae, in traits that lead to resistance or susceptibility to jasmonate (JA)-dependent defences of a host plant but also in traits responsible for induction or repression of JA defences. We characterized three distinct lines of T. urticae that differentially induced JA-related defence genes and metabolites while feeding on tomato plants (Solanum lycopersicum). These lines were also differently affected by induced JA defences. The first line, which induced JA-dependent tomato defences, was susceptible to those defences; the second line also induced JA defences but was resistant to them; and the third, although susceptible to JA defences, repressed induction. We hypothesize that such intraspecific variation is common among herbivores living in environments with a diversity of plants that impose diverse selection pressure.

The PP2C-type phosphatase AP2C1, which negatively regulates MPK4 and MPK6, modulates innate immunity, jasmonic acid, and ethylene levels in Arabidopsis

Wound signaling pathways in plants are mediated by mitogen-activated protein kinases (MAPKs) and stress hormones, such as ethylene and jasmonates. In Arabidopsis thaliana, the transmission of wound signals by MAPKs has been the subject of detailed investigations; however, the involvement of specific phosphatases in wound signaling is not known. Here, we show that AP2C1, an Arabidopsis Ser/Thr phosphatase of type 2C, is a novel stress signal regulator that inactivates the stress-responsive MAPKs MPK4 and MPK6. Mutant ap2c1 plants produce significantly higher amounts of jasmonate upon wounding and are more resistant to phytophagous mites (Tetranychus urticae). Plants with increased AP2C1 levels display lower wound activation of MAPKs, reduced ethylene production, and compromised innate immunity against the necrotrophic pathogen Botrytis cinerea. Our results demonstrate a key role for the AP2C1 phosphatase in regulating stress hormone levels, defense responses, and MAPK activities in Arabidopsis and provide evidence that the activity of AP2C1 might control the plant's response to B. cinerea.

Effect of imidacloprid on the reproduction of acaricide-resistant and susceptible strains of Tetranychus urticae Koch (Acari: Tetranychidae)

Occasional reports linking neonicotinoid insecticide applications to field population outbreaks of the two-spotted spider mite, Tetranychus urticae Koch, have been a topic of concern for integrated pest management (IPM) programmes, particularly in apples. In order to shed light on the factors which may contribute to the occasional field population increase of T. urticae following the application of neonicotinoid insecticides, greenhouse experiments have been set up. Four different T. urticae strains, namely GSS (acaricide-susceptible), WI (organophosphate-selected), USA (a largely uncharacterised strain) and Akita (METI (mitochondrial electron transport inhibitor) acaricide-resistant and cross-resistant to dicofol), were compared for their fecundity without insecticide treatment and for their ovipositional response to foliar and drench applications of the field-relevant dose of imidacloprid (100 mg litre(-1)). Without insecticide treatment, strain GSS laid significantly more eggs (162.50 (+/- 5.43)) than the multiple resistant strain Akita (139.90 (+/- 5.54)) during a 16 day oviposition period. With imidacloprid treatment the highest effect was observed with GSS, with a significantly reduced number of eggs in drench (143.40 (+/- 4.22)) and foliar (144.60 (+/- 5.85)) applications. For strains Akita and USA, no significant differences were observed in oviposition between imidacloprid treatments and controls. The proportion of F1 female offspring decreased significantly with drench application for GSS and WI, while no differences were observed among strains in the survival of F1 immature stages, except for strain USA. The viability of eggs was relatively high (from 82.9 (+/- 4.5)% for USA to 95.2 (+/- 1.2)% for GSS) and not affected by imidacloprid treatments.

Is the cereal rust mite, Abacarus hystrix really a generalist? - Testing colonization performance on novel hosts

The majority of eriophyoid mites are highly host specific and restricted to a narrow range of acceptable host plant species. The cereal rust mite, Abacarus hystrix was considered to be one of a few exceptions among them and has been found to be using a relatively wide host range. Since this species is a vagrant, inhabiting short-lived plants and aerially dispersing, it has commonly been considered to be a host generalist. Here the opposite hypothesis is tested, that host populations of A. hystrix are specialized on their local host plants and may represent host races. For this purpose, females from two host populations (quack grass, Agropyron repens and ryegrass, Lolium perenne) were transferred, and subsequently reared, on their normal (grass species from which females came from) and novel (other grass species) hosts. The female's fitness was assessed by survival and fecundity on the normal and novel host. Females of both populations had no success in the colonization of the novel host. They survived significantly better and had significantly higher fecundity on their normal host than on the novel one. These findings correspond with observations on host-dependent phenotype variability and host acceptance. The presence of locally specialized host populations in A. hystrix may be evidence for high host specificity among eriophyoids and the viruses they transmit. The main conclusion is that A. hystrix, which so far has been considered as a host generalist, in fact may be a complex species consisting of highly specialized host races.

Differential timing of spider mite-induced direct and indirect defenses in tomato plants

Through a combined metabolomics and transcriptomics approach we analyzed the events that took place during the first 5 d of infesting intact tomato (Lycopersicon esculentum) plants with spider mites (Tetranychus urticae). Although the spider mites had caused little visible damage to the leaves after 1 d, they had already induced direct defense responses. For example, proteinase inhibitor activity had doubled and the transcription of genes involved in jasmonate-, salicylate-, and ethylene-regulated defenses had been activated. On day four, proteinase inhibitor activity and particularly transcript levels of salicylate-regulated genes were still maintained. In addition, genes involved in phospholipid metabolism were up-regulated on day one and those in the secondary metabolism on day four. Although transcriptional up-regulation of the enzymes involved in the biosynthesis of monoterpenes and diterpenes already occurred on day one, a significant increase in the emission of volatile terpenoids was delayed until day four. This increase in volatile production coincided with the increased olfactory preference of predatory mites (Phytoseiulus persimilis) for infested plants. Our results indicate that tomato activates its indirect defenses (volatile production) to complement the direct defense response against spider mites.