Jun Tagawa

Sex allocation and clutch size in the gregarious larval endoparasitoid wasp, Cotesia glomerata.

The ability of the gregarious larval endoparasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) to adjust progeny sex ratio and clutch size was investigated. The sex ratios (proportion of males) of field clusters were diverse, but many (70%) were female‐biased. Nearly 10% yielded males only, suggesting a low percentage of unmated females in the field. In over half of the clusters containing females, the sex ratio was below 0.3. Superparasitism was common in the field, and females were believed to increase progeny sex ratio when attacking previously‐parasitized hosts. However, in a single oviposition bout, sex allocation was not precisely controlled both in the field and laboratory. In the laboratory, the number of eggs laid in a day tended to decrease with increasing female age. For females which were offered two hosts per day and for those offered three hosts per day, this value became nearly the same several days after the start of oviposition. The total number of hosts which a female could parasitize during her lifetime was often less than 40. Some of the old females which attacked more than 40 hosts produced male‐biased clutches; this was due to sperm depletion, because sperm remained viable throughout a females lifetime. The amount of sperm used in a single oviposition bout seemed fixed and was not dependent on the number of eggs laid. Females with much oviposition experience did not produce new eggs to compensate for deposited eggs, and the efficiency of egg use (deposited eggs/total eggs) was more than 80%.

Effects of the gregarious parasitoids, Apanteles ruficrus and A. Kariyai, on host growth and development

Abstract Two gregarious parasitoids, Apanteles ruficrus and A. kariyai attack larvae of the common armyworm, Pseudaletia separata . Their growth pattern and growth rate of the parasitoids were not affected by host age at the time of oviposition, even though host weight increased exponentially with age. Consequently, the maximal weight of a single parasitoid larva was nearly constant regardless of host instar parasitized. Parasitoid females laid more eggs in later-instar hosts than in earlier-instar hosts. When parasitized at the same age, heavily parasitized hosts attained a larger mass than lightly parasitized larvae. Therefore, the ratio of the maximal weight of the host to the parasitoid mass was nearly constant. These results indicate that host growth is regulated by the parasitoids.

Different combinations of multiple defence traits in an extrafloral nectary‐bearing plant growing under various habitat conditions

Summary The effects of multiple defence traits on herbivory under different abiotic conditions have not been reported in the literature. To clarify the relations between plant defence traits and abiotic factors in the field, we investigated the plasticity of multiple defence traits in young Mallotus japonicus growing in three habitat conditions: open sites, forest edges and treefall gaps. The plants used multiple defence traits differently in three habitats. At open sites, trichomes, pellucid dots and extrafloral nectaries (EFNs) were expressed better than at the treefall gaps, but food bodies (lipid-rich particles) were fewer. Fewer ants were observed on the plants at open sites than on plants in other habitats. Leaves collected from the open sites were consumed less by the generalist herbivore, Spodoptera litura, than leaves collected from the other habitats. The effect of ants on herbivore exclusion was not readily apparent at open sites. Light intensity at the open sites was high. Soil water contents were low. Tolerance was greater in the forest edge condition, where light intensity and soil water contents were intermediate. Synthesis. The defence tactics of EFN-bearing plants in the field must be determined considering the complex effects of different abiotic factors. Each plant can change its combination of defence traits in response to abiotic habitat conditions.

Effects of mating on the oviposition behaviour of the parasitic wasp,Apanteles glomeratus L. (Hymenoptera: Braconidae)

The oviposition behaviour of mated or virgin females of the parasitic wasp,Apanteles glomeratus L., was investigated. Virgin females laid fewer eggs in a shorter time than did mated females (P<0.01), though the attack ratio and attack time of these 2 female types were the same. Progeny sex ratios of mated females suggested the occurrence of sperm shortage, because old mated females produced clutches of a high male proportion, sometimes consisting of males only. Recopulation of females was also observed, but actual insemination did not occur, and therefore,A. glomeratus females are considered to be fundamentally monogamous.

Ant-Attendance in Extrafloral Nectar-Bearing Plants Promotes Growth and Decreases the Expression of Traits Related to Direct Defenses

In addition to direct defenses, some plant species provide extrafloral nectar (EF-nectar) and/or food bodies (lipid-rich particles) to attract ants for their own indirect defenses. To ascertain why such plants use indirect defenses, we investigated the respective costs of direct and indirect defenses of Mallotus japonicus seedlings grown with and without ants present. Mallotus japonicus plants growing with ants present (ant-present) secreted larger volumes of EF-nectar, containing greater amounts of sugars, as an indirect defense trait. These plants also showed chemical defensive traits, such as the number of pellucid dots and the amount of accumulated phenolics, to a lesser degree than plants without ants (ant-absent) did. Moreover, the ant-present plants grew faster. The estimated amounts of EF-nectar sugars and food bodies were small compared to the amount of phenolics. Plant biomass was correlated negatively with pellucid dot density and phenolic concentration. Plant height was correlated negatively with phenolic concentration. Moreover, leaf biomass was correlated negatively with trichome density. Taken together, these results suggest a tradeoff between the expression of direct defense traits and plant growth. Mallotus japonicus achieves more rapid growth with ants present. We propose that this occurs because these ants provide low-cost indirect defenses allowing plants to re-allocate their energy from direct defenses to growth instead. This mutual benefit apparently facilitates ant–plant defensive mutualism.

Leaf surface preference in the cabbage worm, Pieris rapae crucivora, and parasitism by the gregarious parasitoid Cotesia glomerata

Leaf surface preference of the cabbage worm, Pieris rapae crucivora Boisduval (Lepidoptera: Pieridae), for cabbage, Brassica oleracea L. var. capitata (Brassicaceae), and parasitism by the parasitoid Cotesia glomerata (L.) (Hymenoptera: Braconidae) were investigated experimentally in the laboratory. Female butterflies did not discriminate between the adaxial and abaxial surfaces of cabbage leaves when laying eggs on a vertically placed leaf. Larvae also did not discriminate between the adaxial and abaxial surfaces throughout their larval life. However, second and third instars preferred the lower surface of horizontally placed leaves to the upper surface, irrespective of whether they had hatched on the upper or lower side; other instars showed no preference for the lower surface. Parasitism rates of first and second instars on the upper surface were higher than those of larvae on the lower surface. Egg distribution on leaf surfaces and the leaf surface preference by young larvae are discussed in terms of avoidance of parasitism by the parasitoid C. glomerata.

Aggressiveness of ants attracted to the extrafloral nectary-bearing plant, Mallotus japonicus, and temporal fluctuations in their abundance

Many plants that bear extrafloral nectaries (EFNs) attract various ant species that can exclude herbivores. The aggressiveness of the attracted ants and their temporal activity patterns are important factors that can affect the efficiency of herbivore exclusion from the plant. However, the characteristics of this mutualistic relationship between EFN‐bearing plants and ants have not been sufficiently elucidated. We investigated the aggressiveness of six ant species against the common armyworm, Spodoptera litura Fabr., and temporal fluctuations in the abundance of four aggressive ant species on an EFN‐bearing plant, Mallotus japonicus (L.f.) Müll. Arg. Workers of Crematogaster teranishii Santschi, Pheidole noda Smith, Pristomyrmex punctatus Smith and Formica japonica Motschoulsky were observed to be highly aggressive. In contrast, workers of Camponotus vitiosus Smith showed low aggressiveness. Paratrechina flavipes Smith workers did not attack the herbivore. The activity patterns of the four aggressive ant species greatly differed. Crematogaster teranishii and Ph. noda workers were constantly active throughout the day and night. In contrast, F. japonica was diurnal. Pristomyrmex punctatus was principally nocturnal. Formica japonica workers foraged solitarily, whereas workers of the other three species foraged in a group or recruited nestmates. Our results suggest that the efficacy of the indirect defense in M. japonicus depends principally on the attracted ant species.

Effects of relative humidity on cocoon formation and survival in the braconid wasp Cotesia glomerata

Abstract The effects of relative humidity (RH) on cocoon formation and survival in the braconid parasitoid wasp Cotesia glomerata (L.) (Hymenoptera: Braconidae) are investigated under various humidity conditions (50, 75, 90, 95 and 100% RH) at 20 °C and under an LD 16 : 8 h photoperiod. The mortality rate at the time of egression from hosts under 100% RH is significantly higher than for other RHs. Cocoon clusters formed at 100% RH spread significantly more than those formed at 50, 75, or 90% RH. Developmental periods differ significantly among RHs under which wasps developed. The mean period from the egression from hosts to adult emergence is 8.7 days when developed at 50–95% RHs, and 8.0 days at 100% RH. The emergence rates of C. glomerata that are maintained under the same humidity conditions after egression from hosts are not significantly different among RHs. However, emergence rates from cocoons that are transferred from 100% RH to 50 and 75% RH are < 70%, although the rates are > 90% in most cases. Some wasps do not emerge from cocoons: more than 60% die after adult eclosion at all RHs; the relative frequency of adult deaths is approximately 90% at 50% RH. Relative humidity influences the cluster and cocoon status strongly: both good clusters and cocoons are formed at low RHs. Emergence rates from cocoons of different ranks are significantly different: the rates of low‐rank cocoons are low at low RHs. The survival of C. glomerata is affected strongly by RH through cocoon formation.

Two Mallotus species of different life histories adopt different defense strategies in relation to leaf age

Plants defend their leaves using multiple defense traits that change functions with leaf age. We examined the effects of leaf age on the development of multiple defense traits in two related Mallotus (Euphorbiaceae) species: young plants of the fast-growing Mallotus japonicus (Spreng.) Mull. Arg. and the slow-growing Mallotus philippensis (Lam.) Mull. Arg. Sequential leaves of the two species were measured for their leaf area, leaf mass/area, densities of trichomes and pellucid dots, extrafloral nectar volume, and the numbers of extrafloral nectaries and pearl bodies. Mallotus japonicus shifted its defense tactics from direct defense using trichomes and pellucid dots in young leaves to biotic defense using extrafloral nectar and pearl bodies in middle-aged leaves. In contrast, M. philippensis used direct, chemical defense throughout all leaf ages, together with the shift from indirect, biotic defense using extrafloral nectar in young leaves to direct, physical defense using leaf toughness in middle-aged leaves. These results strongly suggest that, in relation to life history, plants can alter optimal combinations of multiple defense traits with leaf age.

Endogenous periodicity in egg-number fluctuation in Lasius japonicus (Formicidae)

Abstract To confirm oviposition periodicity in queen ants of Lasius japonicus (Formicidae), we performed breeding experiments under four constant conditions: 25, 20, and 17.5 °C under a 12L:12D photoperiod, and 17.5 °C under a 24L photoperiod. After nuptial flight, L. japonicus queens began to lay eggs immediately after the onset of breeding in all conditions. The number of eggs observed in each colony peaked on days 20–40 and then decreased gradually. After the decrease, the egg number began to increase and then decrease again in most colonies. Larval emergence occurred in some colonies but did not occur in many others. Period lengths of cyclic fluctuation in egg number, which were estimated using Fourier analysis, were on average 130–150 days in all four experimental conditions. Larval emergence seemed not to affect the peak days. The decline in queens’ oviposition activity might correspond with that occurring in nature.