Herbivore egg deposition induces tea leaves to arrest the egg‐larval parasitoid Ascogaster reticulata

Plants are able to activate direct and indirect defences against egg deposition by herbivorous insects. A known indirect defence is the production of synomones to help egg‐ and egg‐larval parasitoids to locate their hosts. The wasp Ascogaster reticulata Watanabe (Hymenoptera: Braconidae) is a solitary egg‐larval parasitoid of the moth Adoxophyes honmai Yasuda (Lepidoptera: Tortricidae), which lays eggs and feeds as caterpillars on the leaves of the tea plant Camellia sinensis (L.) Kuntze (Theaceae). Here, we studied whether or not oviposition by A. honmai induces tea plants to produce synomones that help the parasitoid to locate its host. An olfactometer bioassay suggested that synomones produced by the infested plants did not attract the parasitoid over a short range. However, a contact bioassay showed that tea leaves were induced to arrest the parasitoid 24 h after egg deposition and remained induced until the host‐egg masses were no more attractive to the parasitoids. Wing scales and deposits of adult moths and the contents of the egg masses did not induce the tea leaves to arrest the parasitoid, but the contents of the female moth's reproductive system did. Synomone induction was systemic: uninfested leaves in the vicinity of egg‐laden leaves also arrested the parasitoid.     

Sex pheromone production and perception in the mating disruption-resistant strain of the smaller tea leafroller moth, Adoxophyes honmai

Mating disruption is an environmentally safe plant protection strategy that uses a synthetic copy of an insect pheromone to interfere with sexual communication and hence reproduction. To date, a number of pest moths have been controlled with applications of formulated pheromones as mating disruptants. Recently, however, the first example of resistance to mating disruption was documented in one of the major tea pests in Japan, the smaller tea leafroller moth, Adoxophyes honmai Yasuda (Lepidoptera: Tortricidae). To avoid other such cases, it is important to elucidate the mechanism(s) by which the disruptant lost its effectiveness. To this end, we imposed further selection by rearing field‐collected resistant insects with a synthetic pheromone in the laboratory. After more than 70 generations of selection, a strain with quite strong resistance was established, males of which could find and copulate with their mates even in the presence of 1 mg l^?1 of disruptant. Although the mating ability of this strain was greatly increased, the composition and blend ratio of the sex pheromone produced and emitted by females were not obviously changed in comparison with those of females sensitive to mating disruption. However, male response to the pheromone blend was markedly broadened after selection so that resistant males could locate a synthetic pheromone source even when it lacked a pheromone component that is normally necessary for attraction. Males capable of locking onto off‐ratio pheromone blends may be better able to find calling females in pheromone‐treated environments than narrowly tuned males because of greater capability of overcoming sensory imbalance.     

Species‐specific elicitors induce tea leaf to arrest the endoparasitoid Ascogaster reticulata (Hymenoptera: Braconidae)

Ascogaster reticulata Watanabe (Hymenoptera: Braconidae) is an egg‐larval endoparasitoid of the smaller tea tortrix, Adoxophyes honmai Yasuda. Recent studies have examined tritrophic interactions among Camellia sinensis, A. honmai and A. reticulata, but the effect of non‐host insects on the induction of tea plant that may affect foraging behaviour of A. reticulata remains unclear. In this study, we selected two non‐host insects, Homona magnanima Diakonoff and Ostrinia furnacalis (Guenée), as representative species in our bioassays. Tea leaves were treated with homogenized female reproductive tissues of a non‐host insect in comparison with untreated leaves in a choice test. Residence times of parasitoids on both leaves were recorded. The parasitoids seemed to prefer walking on leaves treated with homogenates of H. magnanima over untreated leaves, but the difference in residence times was not significant. In contrast, its residence time on leaves treated with homogenates of O. furnacalis was significantly shorter than that on untreated leaves. Thus, the induction of tea leaf surface chemicals may differ among moth species, which may produce different types of elicitors. This difference may, in turn, affect the host‐searching behaviour of A. reticulata.     

Deterrent effects of intact plants on host-searching behavior of parasitoid wasps

Female parasitoids distinguish between host-infested and intact plants using chemical cues; however, the contribution of intact plants to host searching of parasitoids has not been investigated so far. Here, we tested how host-searching behavior of the parasitoid wasp, Cotesia kariyai (Watanabe) (Hymenoptera: Braconidae), was affected by intact maize plants in a wind tunnel. To determine the best color for material to create a plant model, we observed flight responses of female wasps to paper plant models of four different colors. Wasps tended to land more frequently on green models than other models. Therefore, a green paper model was used for subsequent experiments. In a no-choice test, female wasps showed higher landing rates on a paper plant model treated with herbivore-induced plant volatiles (HIPVs) than on an intact plant. Moreover, in two-choice tests, wasps preferred the plant model with HIPVs over an intact plant with HIPVs. Intact plants seem to deter C. kariyai females. Our findings suggest that information from intact plants also contributes to the host-searching behavior of females in the natural environment.

     

Effect of parasitism byAscogaster reticulatus [Hym.: Braconidae] on growth of the host,Adoxophyes sp. [Lep.: Tortricidae]

The developmental interaction between the egg/larval parasitoid,Ascogaster reticulatus Watanabe (Hymenoptera: Braconidae) and its host,Adoxophyes sp. (Lepidoptera: Tortricidae) was examined. Prior to the egress of a final-instar parasitoid larva from the 4^th-instar host larva, host weight decreased by 22% from the maximum weight. The final body weight of a host larva was 27% of the maximum weight of a healthy 5^th-instar host. Food consumption was significantly reduced in both 3^rd-and 4^th-instar parasitized larvae compared with healthy ones. In the 4^th instar, a parasitized larva consumed 28% less artificial diet and produced less frass than a healthy larva. The growth rate of the endoparasitoid larvae greatly increased after their host's molt to the 4^th instar. Parasitoid larval volume increased 40 fold in the 4^th-instar host.      

Establishment of the culture model system that reflects the process of terminal differentiation of connective tissue-type mast cells

To understand physiological roles of tissue mast cells, we established a culture system where bone marrow-derived immature mast cells differentiate into the connective tissue-type mast cell (CTMC)-like cells through modifying the previous co-culture system with Swiss 3T3 fibroblasts. Our system was found to reproducibly mimic the differentiation of CTMCs on the basis of several criteria, such as granule maturation and sensitivity to cationic secretagogues. The gene expression profile obtained by the microarray analyses was found to reflect many aspects of the differentiation. Our system is thus helpful to gain deeper insights into terminal differentiation of CTMCs.     

Host frass as arrestant chemicals in locating host Mythimna separata by the tachinid fly Exorista japonica

We investigated the response of the tachinid fly, Exorista japonica (Townsend), to host frass or its extracts in order to clarify the host location mechanisms of female flies in a potential host habitat. Host searching time in a patch and the number of patch visits were analyzed by using a frass-containing patch which was excreted by host larvae, Mythimna separata (Walker) (Lepidoptera: Noctuidae), and patches to which host frass extracts were applied. E. japonica females were arrested in response to the host-frass-containing patch after contacting the frass with their front tarsi, thereby spending most of the time to search the patch and to revisit the host-frass patch. While host-searching time in the patch by the females was longest at their first visit of a patch with host frass, searching time decreased with successive visits. The female flies also exhibited area-restricted searching with methanol extracts of the host frass. Area-restricted searching activity increased with the concentration of host-frass extract, i.e., total searching time in the patch and the number of patch visits varied in a dose-dependent manner. E. japonica females likely employ chemicals in host frass as arrestants in host location.     

Host recognition by the egg–larval parasitoid Chelonus inanitus: effects of physical and chemical cues

Parasitoid wasps are widely used as biocontrol agents. For successful deployment, it is important to know how they search for and recognize their hosts. They often use chemical cues in host searching and recognition. Little attention has been paid to the use of physical cues, particularly when combined with chemical cues, though a combination of cues may improve searching efficiency. Chelonus inanitus L. (Hymenoptera: Braconidae) is a solitary, egg–larval parasitoid wasp that parasitizes various lepidopteran species, including serious crop pests. In this study, effects of physical and/or chemical cues on host recognition were investigated in C. inanitus by observing its antennal searching, ovipositor probing, and the duration of arrestment. In a physical cue test, egg-mass models with glass beads differing in size were used, and a single-egg model with a specific size of bead. Egg-mass models with beads of 0.4–1.0 mm in diameter arrested C. inanitus, in accordance with egg size and arrangement of the host. In a chemical cue test, extract from egg masses of the host Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae), equivalent to half of a host egg mass and extract diluted 100-fold were used, and the undiluted extract stimulated C. inanitus. Untreated egg-mass models (0.2-mm-diameter glass beads) and diluted egg extract arrested C. inanitus less effectively; however, when glass beads were treated with the diluted extract, arrestment increased to the same degree as the undiluted extract. These results suggest that C. inanitus uses both physical and chemical cues to recognize eggs of its host, S. litura, and that these stimuli act synergistically.