Yoshiko Ayabe

Spatial distributions of the leafminer Ophiomyia maura (Diptera: Agromyzidae) in host plant Aster ageratoides

The seasonal occurrence and among‐plant and within‐plant spatial distribution of the multivoltine leafminer Ophiomyia maura Meigen (Diptera: Agromyzidae) on the herbaceous plant Aster ageratoides Turcz. subsp. ovatus (Asteraceae) were investigated in the field. O. maura has at least four generations a year and mines per leaf fluctuate with a mean of 0.007 throughout the occurrence period. Seasonal occurrence is associated with abundance of new host leaves, suggesting O. maura females prefer to oviposit in newly emerged leaves. The among‐plant distribution of O. maura is described by a Poisson distribution early in the season but tends to be weakly clumped later. The within‐plant vertical distribution of larval mines increased from middle to upper leaves during plant development, because mined leaves in the middle position early in the season move downward with the emergence of new leaves, shifting mined leaves from the position where O. maura oviposits eggs. Later in the season, mined leaves remain where they are deposited because few new leaves emerge. The spatial distribution of O. maura, resource utilization patterns, and host plant characteristics are discussed.

Directed Search Pattern of a Leafminer Parasitoid Among Mines of Host Larvae

Abstract Parasitoids have developed a variety of searching strategies to maximize their searching efficiency. To increase the efficiency, leafminer parasitoids first search for host mines, which are linear and visually conspicuous, and then search for host larvae situated at the end of the mines by tracking the mines. We hypothesized that the leafminer parasitoids have the ability of directional orientation toward larvae as opposed to orienting randomly and tested this hypothesis by using the leafminer Liriomyza trifolii (Burgess) (Diptera; Agromyzidae) and its parasitoid Hemiptarsenus varicornis (Girault) (Hymenoptera: Eulophidae). Direct observations of parasitoid behavior revealed that female parasitoids mostly selected the correct direction immediately after mine encounter. However, they did not select correctly when the host had been parasitized, or when the point of the mine encounter was far from the host larva, suggesting the possibility of sound-based search. Curiously, the success of encounter rates was not influenced by the directions that the female had selected. This was because the female turned to the correct direction during her search even after selecting the wrong direction. Thus, female H. varicornis can recognize which end of the mine the target larva lays upon mine encounter or during the foraging bout. The importance of the ability of recognizing the correct direction by the parasitoid and the possible mechanism involved in this ability is discussed.

Plasticity in resource use by the leafminer moth Phyllocnistis sp. in response to variations in host plant resources over space and time

We examined how sedentary herbivorous insects cope with changes in resource availability with host plant phenology by studying the multivoltine lepidopteran leafminer Phyllocnistis sp. on the Japanese privet (Ligustrum japonicum). The privet offers three resources: primary shoots in the spring, lammas shoots later in the season, and older leaves. We examined the seasonal abundance of these resources and the density of mines in each to assess competition between leafminers at the tree, shoot, and leaf scale. The leafminer initially used leaves of primary shoots, then shifted to young leaves of lammas shoots, but never used old leaves. The relative scarcity of lammas shoots increased the density of mines, so the leafminers maximized their use of the intra-leaf microhabitat: they commonly preferred the lower surface of leaves, but came to use both surfaces. Thus the leafminer compensated for the lower availability of lammas shoots by maximizing microhabitat use.

Specific Mining Pattern as a Result of Selective Feeding Within a Leaf by the Dipteran Leafminer Ophiomyia maura (Diptera: Agromyzidae)

ABSTRACT Leaf mines exhibit complicated patterns with crosses and branch structures. Mining patterns are hypothesized to be a result of selective feeding on leaf areas that are nutritionally rich or that are poor in chemical and structural defenses (the selective feeding hypothesis). The hypothesis was tested using the leafminer Ophiomyia maura Meigen (Diptera: Agromyzidae) by examining leaf anatomy and nutritional content of the host plant Aster microcephalus (Miq.) Franch. et Savat. variety ovatus (Franch. et Savat.) Soejima et Mot. Ito (Asteraceae). O. maura shows a specific mining pattern by preferentially mining the marginal part of the leaf. Cross sections of mined leaves revealed that O. maura consumed a layer of palisade parenchyma cells. The mining site of O. maura was limited to a particular area by the midrib and lateral veins, but not by minor veins, because midrib and lateral veins have well developed parenchyma cells around the vascular bundles and interrupted the palisade layer. By mining at the marginal part of the leaf, O. maura avoided the midrib and lateral veins, or pinnate venation of A. microcephalus. The nitrogen content of the marginal part of the leaf was higher than that of the inner part of the leaf, which also contributed to O. maura mining the marginal part of the leaf. The specific mining pattern of O. maura demonstrated that the leafminer could have developed an adaptive life-history strategy by responding in a most efficient manner to the arrangement of veins and the nutritional variability even at a within-leaf scale, following the selective feeding hypothesis.

Relationship between radiocesium contamination and the contents of various elements in the web spider Nephila clavata (Nephilidae: Arachnida).

The accident at the Fukushima Dai-ichi nuclear power plant seriously contaminated a large area in northeast Japan with a large amount of radioactive material. Consequently, various organisms, including arthropods, in the ecosystem have been contaminated with radiocesium ((137)Cs) through the food chain. We previously showed that the web spider Nephila clavata was contaminated with (137)Cs and that the level of contamination, which varied among spider individuals, was independent of the amount of prey consumed. The present study aimed to clarify the mechanisms that could determine the level of (137)Cs contamination in N. clavata. We first demonstrated the patterns of contents of over 30 elements in N. clavata that were collected at two forest sites (PS and ES) in Fukushima and then focused on the relationships between the contents of the alkali metals Li, Na, K, and Rb and the accumulation of (137)Cs in the spiders; Cs is an alkali metal and is expected to act similarly to Li, Na, K, and Rb. We also focused on the content of the non-alkali element, Cu, which is an essential element for oxygen transport in spiders. We found that Na content correlated positively with (137)Cs accumulation at both sites, which suggested that (137)Cs accumulation in N. clavata was related with the dynamics of Na. The K-, Rb-, and Cu-(137)Cs relationships were site specific; the relationships were significant at site PS, but not significant at site ES. Factors causing the site specific relationships and the probable pathway for (137)Cs transfer from soil to plants and then to higher trophic levels are discussed in terms of the transfer processes of the alkali metals.

Complex Feeding Tracks of the Sessile Herbivorous Insect Ophiomyia maura as a Function of the Defense against Insect Parasitoids

Because insect herbivores generally suffer from high mortality due to their natural enemies, reducing the risk of being located by natural enemies is of critical importance for them, forcing them to develop a variety of defensive measures. Larvae of leaf-mining insects lead a sedentary life inside a leaf and make conspicuous feeding tracks called mines, exposing themselves to the potential risk of parasitism. We investigated the defense strategy of the linear leafminer Ophiomyia maura Meigen (Diptera: Agromyzidae), by focusing on its mining patterns. We examined whether the leafminer could reduce the risk of being parasitized (1) by making cross structures in the inner area of a leaf to deter parasitoids from tracking the mines due to complex pathways, and (2) by mining along the edge of a leaf to hinder visually searching parasitoids from finding mined leaves due to effective background matching of the mined leaves among intact leaves. We quantified fractal dimension as mine complexity and area of mine in the inner area of the leaf as interior mine density for each sample mine, and analyzed whether these mine traits affected the susceptibility of O. maura to parasitism. Our results have shown that an increase in mine complexity with the development of occupying larvae decreases the probability of being parasitized, while interior mine density has no influence on parasitism. These results suggest that the larval development increases the host defense ability through increasing mine complexity. Thus the feeding pattern of these sessile insects has a defensive function by reducing the risk of parasitism.

Avoidance of parasitoid attack is associated with the spatial use within a leaf by a lepidopteran leafminer

In prey‐predator systems, top‐down effects can be a powerful determinant for spatial distributions of prey through their search for enemy‐free space. Leafminers live and eat within leaves, making feeding tracks called mines, and mine conspicuousness exposes them to a high risk of parasitism. Those lepidopteran leafminers that use lower leaf surfaces as mining sites show wide evolutionary radiation. We hypothesized that leafminers making mines on the lower surface are less often detected by parasitoids and thus have a selective advantage in avoiding parasitism compared to those on the upper surface. To investigate the adaptiveness of lower‐surface mining, we examined the relationship between parasitism and within‐leaf mine distribution for 3 years using a field population of the leafminer Phyllocnistis spec. (Lepidoptera: Gracillariidae, Phyllocnistinae), which prefers the lower surface of leaves of the Japanese privet, Ligustrum japonicum Thunb. (Oleaceae). Parasitoid attack was more frequent in the upper‐surface mines than in the lower‐surface mines and on leaves with upper‐surface mines than on leaves with only lower‐surface mines. When both surfaces were mined, leafminers on the lower surface could avoid parasitism. Upper‐surface mines were attacked by more parasitoid species as compared to lower‐surface mines. Although the results demonstrated that mining on the lower surface was advantageous in avoiding parasitism, the vulnerability of lower‐surface mines to parasitism varied depending on their abundance. When many lower‐surface mines were present, lower‐surface mines suffered a higher parasitism rate than upper‐surface mines, probably because parasitoids formed search images for and concentrated on lower‐surface mines. This study suggests that the preferential use of the lower leaf surface by leafminers is in part attributed to interactions with parasitoids.

Invertebrate assemblages of hanging and container litter on conifer trees

ABSTRACT Arboreal litter provides a habitat patch for detritus-based invertebrates in the aboveground part of forest ecosystems and exists in two forms. Hanging litter (HL) remains attached to branches and tree trunks after foliage dies, or hangs as intercepted falling litter exposed to the surrounding air. In comparison, litter in container habitats (CL) is deposited in pocket-like structures, such as treeholes and trunk crevices, that are enclosed by woody substrates. In experiments using litter bags and artificial containers, we: (1) compared litter invertebrate assemblages among HL, CL, and forest-floor litter, and (2) evaluated the relative importance of patch size and resource quantity for litter invertebrate assemblages in HL on Japanese cedar (Cryptomeria japonica) trees. Our results showed that the moisture content of litter and collembolan density tended to be higher in arboreal container litter (i.e. CL) than in arboreal bag litter (i.e. HL). In the second experiment, the total abundance of invertebrates was influenced more by resource quantity (litter weight) than by patch size (litter bag area). Redundancy analysis (RDA) indicated that each major group (oribatid mites, collembolans, and Gamasida) was influenced by different environmental factors in the HL. Our findings indicate that: (1) HL and CL represent different habitat types for litter invertebrates; (2) the major invertebrate groups have different habitat preferences in the HL; and (3) the abundance of invertebrates in HL is influenced more by litter quantity than by litter density (litter weight per area).

Influence of patch size and resource quantity on litter invertebrate assemblages in dry treeholes

ABSTRACT Treeholes are dry or water-filled cavities that occur in the aboveground parts of forest ecosystems and support terrestrial and aquatic invertebrates. Compared with aquatic invertebrate assemblages in water-filled treeholes, little is known about how strongly patch size or resource quantity affects the terrestrial invertebrate assemblages in dry treeholes. Therefore, we examined the factors that affect terrestrial invertebrate assemblages in dry treeholes using a field experiment with artificial treeholes and by investigating natural treeholes, both in broad-leaved forests in central Japan. In the artificial treeholes, final litter density (final litter weight per unit treehole volume), which indicates the extent of resource concentration, was positively correlated with total invertebrate abundance and negatively correlated with the proportion of litter remaining at the end of the experiment. These results suggest that greater litter density results in an increase in both invertebrate abundance and litter decomposition. In natural treeholes, however, the total abundance of terrestrial invertebrates was not significantly correlated with litter density, litter weight, or treehole volume. The quantity of litter, which consisted mainly of fine particulate organic matter, increased with treehole opening size (area) and volume, suggesting that already decomposed litter was filled, depending on the treehole volume. This may have been attributed to the nonsignificant results concerning the abundance of terrestrial invertebrates in natural conditions. Thus, we suggest that both litter weight and density are important factors in determining the abundance of invertebrate in dry treeholes, even though their influence is weakened in natural treeholes by the effects of other factors.

Oviposition site selection by a lepidopteran leafminer in response to heterogeneity of leaf surface conditions: structural traits and microclimates

1. Leafminer larvae are sedentary and make feeding tracks called mines. Their spatial distribution in trees affects their growth and survival through interaction with the heterogeneity of environments, such as leaf traits and microclimate. Lepidopteran leafminers that mine lower leaf surfaces have shown evolutionary radiation, suggesting that lower surfaces improve leafminer performance.