Keiko Oku

Nonlethal indirect effects of a native predatory mite, Amblyseius womersleyi Schicha (Acari:Phytoseiidae), on the phytophagous mite Tetranychus kanzawai Kishida (Acari: Tetranychidae)

We experimentally tested the indirect and direct effects of Amblyseius womersleyi on Tetranychus kanzawai. The presence of A. womersleyi indirectly reduced egg production of T. kanzawai by 25.9%, although this effect had less impact than direct egg predation. The mechanism of this indirect effect could be explained by behavioral changes in T. kanzawai females; in the presence of A. womersleyi, T. kanzawai females allocated more time to seeking refuge on webs at the expense of feeding on leaves.

Spider mite's use of a refuge during the quiescent stage in the presence of a predator

In the presence of predators, some organisms alter their behavior & morphology to reduce the risk of predation (e. g., Dodson, 1989; Tollrian 1995; Wiackowski & Staronska, 1999; Buskirk & McCollum, 2000). Some preys utilize refuges that they do not inhabit in the absence of predators (e.g., Crowder et al., 1997). Such changes are examples of predator-induced defenses. If an inducible defense had no associated costs, it would be expressed regularly rather than only in specific situations (Riessen, 1992). Inducible defenses are therefore assumed to incur fitness costs that are outweighed by the benefit of predator avoidance. The development of the Kanzawa spider mite, Tetranychus kanzawai Kishida (Acari: Tetranychidae), consists of active stages (larva, protonymph, and deutonymph) and quiescent stages (egg, protochrysalis, deutochrysalis, and teleiochrysalis). Individuals are more likely to be attacked by predators during the quiescent stages than during the active stages (see T. urticae [Shimoda et al., 1997], for example). Tetranychus kanzawai constructs highly complicated webs over the surface of a leaf (Saito, 1983). Empirical observations show that T. kanzawai enters its quiescent stages either on the web, or on the leaf surface (hereafter referred to as ‘on the leaf ’) (Figure 1). Although the space between the web and the leaf can serve as a refuge from some predators, other predators can access this space (McMurtry & Johnson, 1966). Some predatory mites are attracted to spider mite webbing (Pratt & Croft, 1999), and prefer to reside and oviposit on leaf surfaces with the webbing (Roda et al., 2001); Amblyseius womersleyi Schicha (Acari: Phytoseiidae), a predator of T. kanzawai that is native to Japan, is an example of such a predator (K. Oku, pers. obs.). We hypothesized that T. kanzawai enters into quiescent stages on its web as a predator-induced defense. We discuss the benefit and possible costs associated with such a defense.

Genetic differentiation between resistance phenotypes in the phytophagous flea beetle, Phyllotreta nemorum.

Abstract The flea beetle Phyllotreta nemorum L. (Coleoptera: Chrysomelidae) is genetically polymorphic for resistance against the defences of one of its host plants, Barbarea vulgaris R.Br. (Brassicales: Brassicaceae). Whereas resistant flea beetles are able to use B. vulgaris as well as other cruciferous plants as food, non-resistant beetles cannot survive on B. vulgaris. This limitation to host plant use of non-resistant beetles could potentially lead to asymmetric gene flow and some degree of genetic isolation between the different resistance-genotypes. Therefore, we studied the extent of genetic differentiation at neutral allozyme loci between samples of flea beetles that were collected at different locations and first tested for resistance phenotype. Since earlier work has shown a weak, but significant, effect of geographical distance between the samples on their genetic differentiation, in the present study variation at the neutral allozyme loci in P. nemorum was partitioned between geographical distance and resistance-phenotype. Both sources independently contributed statistically significantly to population differentiation. Thus, there appears to be a limitation to genetic exchange between the resistant and non-resistant flea beetles when corrections are made for their geographic differentiation. This is consistent with the presence of some degree of host race formation in this flea beetle.

Host plant acceptance by the phytophagous mite Tetranychus kanzawai Kishida is affected by the availability of a refuge on the leaf surface

We used 11 wild plants to experimentally test the effects of leaf trichomes and leaf quality on host plant acceptance by a phytophagous spider mite, Tetranychus kanzawai, in the absence of predators. There was a positive correlation between leaf hair traits (height/density) and host plant acceptance. We used two model plants (Phaseolus vulgaris and Phaseolus lunatus; the former had higher and denser leaf hairs than the latter) to examine the effects of leaf hairs on the dispersal and fecundity of T. kanzawai in the presence of a predatory mite, Neoseiulus womersleyi. In the presence of N. womersleyi, significantly fewer T. kanzawai females dispersed from a P. vulgaris leaf than from a P. lunatus leaf. Moreover, in the presence of the predator the fecundity of T. kanzawai females on P. lunatus was significantly lower than on P. vulgaris, although in terms of host quality the two plants were equivalent. In the presence of N. womersleyi, T. kanzawai females on P. vulgaris spent more time on webs than those on P. lunatus. Moreover, webs seemed to be less accessible than leaf surfaces to the predator. These results suggest that leaf hairs provide a refuge for T. kanzawai adult females.

The presence of webbing affects the oviposition rate of two-spotted spider mites, Tetranychus urticae (Acari: Tetranychidae)

Several species of tetranychid mites including Tetranychus urticae Koch (Acari: Tetranychidae) construct complicated three-dimensional webs on plant leaves. These webs provide protection against biotic and abiotic stress. As producing web is likely to entail a cost, mites that arrive on a leaf with web are expected to refrain from producing it, because they will gain the benefit of protection from the existing web. Mites that produce less web may then allocate resources that are not spent on web construction to other fitness-enhancing activities, such as laying eggs. To test this, the oviposition rate of T. urticae adult females was examined on leaves with web. As a control, we used leaves where the web had been removed, hence both types of leaves had been exposed to conspecifics previously and were thus damaged. On leaves with web, the oviposition rate of T. urticae females was higher than on leaves where the web had been removed. Therefore, the presence of web constructed by conspecifics enhanced the oviposition rate of T. urticae females. This provides indirect evidence that mites use the web constructed by conspecifics and thereby save resources that can be allocated to other traits that enhance reproductive success.

Mating Strategies of Tetranychus kanzawai (Acari: Tetranychidae) in Relation to Mating Status of Females

Abstract To clarify whether adult male spider mite Tetranychus kanzawai Kishida can recognize the mating status of conspecific females, male preference for virgin and mated females was tested under dual choice conditions. Thirty of 36 males approached virgin females first, indicating that males can perceive female mating status. Males preferred trails made by virgin females to those made by mated females (the ratio was 19:2). Moreover, males also probably used odors to discriminate the mating status of females. These abilities of males may play an important role in gaining access to virgin females. Alternatively, behavior of adult females varied with their mating status. Virgin females were more gregarious and remained on infested kidney bean leaves for a longer time than mated females. This behavior is likely to increase the mating opportunities of virgin T. kanzawai females.

Female mating strategy during precopulatory mate guarding in spider mites

In some taxa, females choose their mates indirectly by using male combat. In the Kanzawa spider mite, Tetranychus kanzawai, adult males guard prereproductive quiescent females. In a dual choice experiment, more males first approached females already guarded by a conspecific male than approached solitary females. In further experiments, I examined which factors attracted males during precopulatory mate guarding. Males were not attracted to males not in a mate-guarding position. In contrast, the presence of a nonbreeding individual, a juvenile male or a female in a mate-guarding position did attract conspecific males. These results suggest that the presence of any conspecific individual in a mate-guarding position stimulates quiescent females to produce chemicals that attract males. Guarded quiescent females also attracted the attention of the predatory mite, Neoseiulus womersleyi, but solitary females did not. Since N. womersleyi search for prey using chemical cues, guarded females that release more chemicals than solitary females are probably in more danger. However, males were not attracted to guarded quiescent females that had been previously exposed to N. womersleyi. Thus, quiescent females appeared to control the release of their chemicals, that is, guarded quiescent females release a male-attractant signal. When more than one male attempts to guard one female, male combat often occurs. I discuss the possibility of indirect mate choice by T. kanzawai females during precopulatory mate guarding facilitated by the use of male-attractant signals.

Effects of predation risk on mating behavior of the Kanzawa spider mite

In the Kanzawa spider mite, Tetranychus kanzawai, adult males locate pre-reproductive quiescent females and engage in precopulatory mate guarding. We found that re-reproductive quiescent females preferred to be near veins, rather than other leaf parts, and moreover, adult males spent more time along the vein than on other parts. Consequently, T. kanzawai males found more quiescent females along veins than those on other parts. However, the predatory mite Neoseiulus womersleyi also found more quiescent T. kanzawai females along veins than those on other parts. Moreover, N. womersleyi found more guarding males than solitary males of T. kanzawai. Thus, we experimentally examined the effects of predation risk on the mating behavior of T. kanzawai. The presence of N. womersleyi reduced T. kanzawai female preference for vein vicinity as a quiescent site. Although the predation risk of guarding T. kanzawai males was lower than that of solitary males after detection by predators, the presence of N. womersleyi also reduced the proportion of guarding T. kanzawai males. These results suggest that the possible benefits of preferring vein vicinity as quiescent sites by T. kanzawai females is outweighed by predation risk in the presence of predators, and that the risk of detection by predators would be more important for T. kanzawai males than the risk of being preyed upon.

Sexual selection and mating behavior in spider mites of the genus Tetranychus (Acari: Tetranychidae)

As sexual selection is a coevolutionary process between males and females, various morphological and behavioral traits have evolved in each sex. In the tetranychid mites Tetranychusurticae Koch and T. kanzawai Kishida (Acari: Tetranychidae), males can mate repeatedly, whereas females normally accept only the first copulation for fertilization. Since early times, it had been reported that males engage in precopulatory mate guarding and combat against conspecifics for females to enhance their reproductive success. On the other hand, it was believed that females do not have opportunities to choose their mates. In the last 10 or so years, however, several new findings related to mating behavior were reported. Some of the findings reinforce our established knowledge, whereas some of them explode it. Here, I review the mating behavior of T. urticae and T. kanzawai by incorporating recent findings and then propose a new direction for future research.

Spider Mites (Acari: Tetranychidae) Deform Their Host Plant Leaves: An Investigation from the Viewpoint of Predator Avoidance

Abstract On two wild host plants, Calystegia japonica Choisy (Convolvulaceae) and Cayratia japonica (Thunb.) Gagnep. (Vitaceae), more tetranychid mites were found on curved leaves than on flat leaves. In addition, more Cay. japonica leaflets inoculated with Tetranychus kanzawai Kishida (Acari: Tetranychidae) females became curved than those without the mite, suggesting that T. kanzawai actively deforms the leaves of its host plants. In the presence of the predatory mite Neoseiulus womersleyi Schicha (Acari: Phytoseiidae), the fecundity of T. kanzawai was indirectly reduced. However, this negative effect of predators was mitigated on curved leaves compared with that on flat leaves. In the presence of predators, T. kanzawai females on curved leaves laid more eggs on webs than those on flat leaves, suggesting that curved leaves provide more refuge for T. kanzawai, which could be a benefit of leaf deformation.