Yukie Sato

Different expression profiles of jasmonic acid and salicylic acid inducible genes in the tomato plant against herbivores with various feeding modes

We compared the expression profiles of jasmonic acid (JA)-inducible genes (Pin2 and LapA1) and salicylic acid (SA)-inducible genes (PRb-1b and GluB) in the tomato (Solanum lycopersicum cv. Micro-Tom) against herbivores using differing feeding modes: the leaf-chewing larvae of the insects Spodoptera litura and S. exigua; the western flower thrips (Frankliniella occidentalis) and the two-spotted spider mite (Tetranychus urticae) as cell-content feeders; and the leaf miner fly (Liriomyza sativae). Feeding by larvae of both S. litura and S. exigua chiefly activated JA-inducible genes, similar to the response to wound stimuli. Feeding by the thrips F. occidentalis also activated JA-inducible genes, as previously reported in Arabidopsis. Feeding by the spider mite T. urticae activated a JA-inducible LapA1 gene but did not activate a JA-inducible Pin2 gene and additionally activated SA-inducible genes, which were accompanied by the accumulation of SA. This may be a strain that represses induction of the JA signaling pathway. One day after oviposition by the leaf miner fly, L. sativae, JA-inducible genes were activated. However, after the L. sativae larvae hatched and began eating within the leaf tissues, JA-inducible gene expression decreased and SA-inducible gene expression increased. Activation of SA-inducible genes (PRb-1b and GluB) by L. sativae larval feeding seems to suppress JA-mediated plant defense but appears to be unrelated to SA accumulation.

Testing for reproductive interference in the population dynamics of two congeneric species of herbivorous mites

When phylogenetically close, two competing species may reproductively interfere, and thereby affect their population dynamics. We tested for reproductive interference (RI) between two congeneric haplo-diploid spider mites, Tetranychus evansi and Tetranychus urticae, by investigating their interspecific mating and their population dynamics when they competed on the same plants. They are both pests of tomato, but differ in the host plant defences that they suppress or induce. To reduce the effect of plant-mediated interaction, we used a mutant tomato plant lacking jasmonate-mediated anti-herbivore defences in the competition experiment. In addition, to manipulate the effect of RI, we introduced founder females already mated with conspecific males in mild RI treatments or founder, virgin females in strong RI treatments (in either case together with heterospecific and conspecific males). As females show first-male sperm precedence, RI should occur especially in the founder generation under strong RI treatments. We found that T. urticae outcompeted T. evansi in mild, but not in strong RI treatments. Thus, T. evansi interfered reproductively with T. urticae. This result was supported by crossing experiments showing frequent interspecific copulations, strong postmating reproductive isolation and a preference of T. evansi males to mate with T. urticae (instead of conspecific) females, whereas T. urticae males preferred conspecific females. We conclude that interspecific mating comes at a cost due to asymmetric mate preferences of males. Because RI by T. evansi can improve its competitiveness to T. urticae, we propose that RI partly explains why T. evansi became invasive in Europe where T. urticae is endemic.

Alternative phenotypes of male mating behaviour in the two-spotted spider mite

Severe intraspecific competition for mates selects for aggressive individuals but may also lead to the evolution of alternative phenotypes that do not act aggressively, yet manage to acquire matings. The two-spotted spider mite, Tetranychus urticae, shows male mate-guarding behaviour and male–male combat for available females. This may provide opportunity for weaker males to avoid fighting by adopting alternative mating behaviour such as sneaker or satellite tactics as observed in other animals. We investigated male precopulatory behaviour in the two-spotted spider mite by means of video-techniques and found three types of male mating behaviour: territorial, sneaker and opportunistic. Territorial and sneaker males associate with female teleiochrysales and spend much time guarding them. Territorial males are easily disturbed by rival males and engage themselves in fights with them. However, sneaker males are not at all disturbed by rival males, never engage in fights and, strikingly, never face attack by territorial males. Opportunistic males wander around in search of females that are in the teleiochrysalis stage but very close to or at emergence. To quickly classify any given mate-guarding male as territorial or sneaker we developed a method based on the instantaneous response of males to disturbance by a live male mounted on top of a brush. We tested this method against the response of the same males to natural disturbance by two or three other males. Because this method proved to be successful, we used it to collect territorial and sneaker males, and subjected them to morphological analysis to assess whether the various behavioural phenotypes are associated with different morphological characters. However, we found no statistical differences between territorial and sneaker males, concerning the length of the first legs, the stylets, the pedipalps or the body.

The parapatric distribution and contact zone of two forms showing different male-to-male aggressiveness in a social spider mite, Stigmaeopsis miscanthi (Acari: Tetranychidae)

Two forms showing different male-to-male aggressiveness, different male morphologies and different diapause attributes are known in Stigmaeopsis miscanthi (Saito), a social spider mite infesting Chinese silver grass (Miscanthus sinensis Anderss). Reproductive isolation exists between the forms, although it is not always complete, and the details of their distributional patterns are unknown, but expected to be parapatric. We searched for the contact zone at Mt. Unzen (Nagasaki Pref., Japan) where both forms are known to occur at different altitudes. We found the two forms together in several M. sinensis stands, suggesting there is frequent contact between the forms in their boundary area. We discuss the mechanism(s) that maintain the parapatry related to their frequent contact and the pattern of reproductive isolation between them.

Function of the web box as an anti-predator barrier in the spider mite, Schizotetranychus recki

Spider mites inhabiting Sasa bamboo show considerable variation in traits believed to be the result of coevolution between predator and prey. In Schizotetranychus recki Ehara inhabiting the hairy leaves of a dwarf bamboo, Sasa senanensis, all quiescent stages, including eggs, appear within web boxes in the leaf hairs of their host plant, and this habit is thought to be a trait involved in predator avoidance. To test this hypothesis, the survival rates of S. recki eggs inside a web box and those freed of a web box were assessed in relation to six predator species that co-occur with the spider mite in the field. The results clearly show that the webbing behavior (web box) of S. recki has a function in avoiding five predator species. However, one predator species, Agistemus summersi Ehara, preyed more on eggs protected by a web box. This suggests that this predator species has a special trait to overcome the nest barriers.

Male-male aggression peaks at intermediate relatedness in a social spider mite

Abstract Theory predicts that when individuals live in groups or colonies, male–male aggression peaks at intermediate levels of local average relatedness. Assuming that aggression is costly and directed toward nonrelatives and that competition for reproduction acts within the colony, benefits of aggressive behavior are maximized in colonies with a mix of related and unrelated competitors because aggression hurts nonkin often, thereby favoring reproduction of kin. This leads to a dome-shaped relation between male–male aggression and average relatedness. This prediction has been tested with bacteria in the laboratory, but not with organisms in the field. We study how male–male aggression varies with relatedness in the social spider mite Stigmaeopsis miscanthi. We sampled 25 populations across a wide geographic range between Taiwan and Japan, representing a gradient of high to low within-population relatedness. For each population the weaponry of males was measured as the length of the first pair of legs, and male–male aggression was tested by placing pairs of nonsibling males together and scoring the frequency of male death over a given period. As these two morphological and behavioral variables correlate strongly, they both reflect the intensity of male–male conflict. Our data on the social spider mite show that male–male aggression as well as weapon size strongly peak at intermediate, average relatedness, thereby confirming theoretical predictions. Inclusive fitness theory predicts that when individuals live in groups or colonies, aggression should peak at intermediate levels of average relatedness in the colony. Here, we study how male–male aggression varies with average relatedness in naturally occurring colonies of the social spider mite Stigmaeopsis miscanthi. In support of theory, male–male aggression and weapon size strongly peak at intermediate average relatedness.

Variation in counterattack effect against a phytoseiid predator between two forms of the social spider mite, Stigmaeopsis miscanthi

In Japan, Stigmaeopsis miscanthi (Saito) occurs in two forms that are differentiated by the intensity of male-to-male aggression, i.e. there is a low aggression and a high aggression group (known as the LW and HG forms, respectively). The effects of counterattack behavior against predators were experimentally compared between the two forms. Parental males and females of both forms could achieve significant counterattack success against Typhlodromus bambusae, a specific predator, and counterattack efficiency increased significantly with parental density. Furthermore, the HG form showed a stronger tendency than the LW form to kill predator larvae. Thus, variation in counterattack success may exist between these two forms, and there is some correspondence between male-to-male belligerence and counterattack effectiveness against predators.

Can the extremely female‐biased sex ratio of the social spider mites be explained by Hamilton’s local mate competition model?

Abstract 1. Extremely female‐biased sex ratios are known in the social spider mite species, Stigmaeopsis longus and S. miscanthi. Whether Hamilton’s local mate competition (LMC) theory can explain such sex ratios was investigated.

Risk assessment of non-target effects caused by releasing two exotic phytoseiid mites in Japan: can an indigenous phytoseiid mite become IG prey?

Two exotic phytoseiid mites, Neoseiulus cucumeris and Amblyseius swirskii, are commercially available in Japan for the control of thrips and other pest insects. As part of a risk assessment of the non-target effects of releasing these two species, we investigated intraguild predation (IGP) between these exotic phytoseiid mites and an indigenous phytoseiid mite Gynaeseius liturivorus, which is promising as an indigenous natural enemy for the control of thrips in Japan. To understand IGP relations between the exotic and indigenous phytoseiid mites after use of the exotic mites for biological control, we investigated IGP between them in the absence of their shared prey. When an IG prey was offered to an IG predator, both exotic and indigenous females consumed the IG prey at all immature stages (egg, larva, protonymph, deutonymph), especially at its larval stages. The propensity for IGP in a no-choice test was measured by the survival time of IG prey corrected using the survival time of thrips offered to the IG predator. There was no significant difference in the propensity for IGP between N. cucumeris and G. liturivorus, but the propensity was significantly higher in A. swirskii than G. liturivorus. The propensity for IGP in a choice test was measured by the prey choice of the IG predator when a conspecific and a heterospecific larva were offered simultaneously as IG prey. Both exotic females consumed the heterospecific larva only. The indigenous female preferentially consumed the heterospecific larva when the heterospecific larva was N. cucumeris, but consumed the conspecific larva when the heterospecific larva was A. swirskii. We concluded that further investigation would be necessary for the exotic mites’ risk assessment, since the propensity for IGP of the two exotic females was similar to or higher than that of the indigenous female in both the no-choice and choice tests.

Why do males choose heterospecific females in the red spider mite

In some species, males readily show courtship behaviour towards heterospecific females and even prefer them to females of their own species. This behaviour is generally explained by indiscriminate mating to acquire more mates, but may partly be explained by male mate preference mechanisms that have developed to choose among conspecific females, as male preference for larger females causes mating with larger heterospecific females. Recently, we found that males of the red spider mite, Tetranychus evansi collected from Spain (invasive population), prefer to mate with females of the two-spotted spider mite, T. urticae rather than with conspecific females. In spider mites, mate preference for non-kin individuals has been observed. Here, we investigated if T. evansi males collected from the area of its origin (Brazil) also show preference for heterospecific females. Secondly, we investigated if mate preference of T. evansi males for heterospecific females is affected by their relatedness to conspecific females which are offered together with heterospecific females. We found that mate preference for heterospecific females exists in Brazilian T. evansi, suggesting that the preference for heterospecific females is not a lack of evolved premating isolation with an allopatric species. We found that T. evansi males showed lower propensity to mate with heterospecific females when alternative females were non-kin in the two iso-female lines collected from Brazil. However, the effect of relatedness on male mate preference was not significant. We discuss alternative hypotheses explaining why T. evansi males prefer to mate with T. urticae females.