Yosiaki Itô

The evolutionary biology of sterile soldiers in aphids

The discovery of a sterile soldier caste in aphids by Shigeyuki Aoki offered behavioural ecologists new and interesting subjects for the study of insect sociality. Quantitative studies on the behaviour of soldiers and the population and genetic consequences are, however, scant. This review summarizes the distribution and peculiarities of sterile soldiers in different aphid taxa and also reports on phenomena that may relate to the evolution of aphid soldiers.

On the methods for determining density-dependence by means of regression

SummaryThe determination of density-dependence of a mortality process is attempted by taking the linear regression of the logarithm of population density (or k defined by Varley and Gradwell, 1960) against the logarithm of previous density, based on the assumption that the slope, b, of the line is smaller than unity for log density-log density relationship or larger than zero for k-log density relationship. It was concluded, however, that the following three factors violate the basic assumption, providing no density-dependence.1.In a Morris plot based on serial data, the value of b tends to be near the value of r. Thus, when the relationship is strongly affected by chance factors, giving remarkably scattered points on graph, the value of b tends to be always lower than unity.2.When the independent variables (log previous density) are subject to sampling error, the value of b tends to be smaller than unity for density-density relations or larger than zero for k-density relations.3.In Morris plot, where log densities are used twice as an independent and a dependent variables excepting the first and the last generation, the effect of timelag strongly reduces the value of b when the number of generations is not large.

Biology ofHyphantria cunea Drury (Lepidoptera: arctiidae) in Japan. V. Preliminary life tables and mortality data in urban areas

Studies on the population dynamics of the fall webworm,Hyphantria cunea have been carried out at three survey stations and along selected roads in the urban area of Tokyo since 1966. Twelve survivorship curves obtained during two years and 8 life tables show that the mortality rate in early developmental stages of the fall webworm is remarkably low as compared with that of other lepidopterous defoliaters and the mortality rate in later developmental stages is compensatory high. The low mortality rate in early stages is considered to be due to the protective role of the nest-web and the lack of egg and larval parasites. All but one parasitic species emerge from prepupae and pupae. Spiders living in the nest-web of the fall webworm play an important role in reducing the number of young larvae. Direct observations and caging experiments showed that relatively high mortality during later larval stages is mainly due to predation by birds (in the first generation) and wasps (in the second generation). The generation mortality in the survey stations always exceeded the level where the population is kept at the steady state, and the outbreak of this moth is considered to be continued by the immigration of adults from large trees growing in gardens on which the larvae can escape from predation pressure.

Decrease in respiratory rate in a wolf spider,Pardosa astrigera (L. Koch), under starvation

Effects of starvation on the suryival period and the respiratory rate in adults of a wolf spider,Pardosa astrigera (L. Koch), were investigated. The spiders used were divided into four groups: well-fed, starved and two limited food groups; in the latter two, each spider was supplied with one leafhopper every second or third day. Adult males and females ofP. astrigera could survive for a long time; 28.8±2.7 days and 54.4±18.9 days, respectively, without any food. The longevities shown here were 73.8% for males and 78.6% for females of those of well-fed spiders, indicating thatP. astrigera adults have a strong tolerance to starvation. The respiratory rate of well-fed adults showed no tendency to increase or decrease with their aging; the mean respiratory rates were 4.86×10−4 mg CO2/mg f.w. (fresh body weight)/hr for males and 3.80×10−4 mg CO2/mg f.w./hr for females. The respiratory rates of starved spiders increased during the first two days of starvation but decreased markedly from the third to the twelfth day, and thereafter retained an almost constant level for each sex. The mean respiratory rates after the twelfth day of starvation were 2.49×10−4 mg CO2/mg f.w./hr for males and 2.76×10−4 mg CO2/mg f.w./hr for females; these values were respectively 48.4% and 63.0% of those prior to starvation. The fresh body weight of starved spiders decreased linearly with time but the rate was small. The respiratory rates of the limited food groups tended to decline with time and thereby their weight losses were minimized. The decrease in the respiratory rate under starvation was considered not to be due to spider exhaustion or senescence but due to an intrinsic change in behaviour and/or metabolism, because when the spiders were supplied with ample food for five days after starvation, the respiratory rate and the body weight rapidly recovered to near the levels prior to starvation. It is suggested that starved spiders use a higher ratio of fat as catabolic substrate than normally fed or satiated ones. Feeding strategies of poikilo-therm predators are discussed.

Preliminary studies on the respiratory energy loss of a spider,Lycosa pseudoannulata

To elucidate the basic food requirement of spiders, the important polyphagous predators of rice-plant insect pests, an attempt was made to measure the respiratory energy loss of fasting spiders,Lycosa pseudoannulata. Relationship between fresh (y) and dry (x) weights of spiders inhabiting the bottom layer of the rice-plant community was represented by the following allometric equation:y=0.428x 0.872. The carbon dioxide production by previously fed and unfed females under the dark at 29°C 100% R. H. was measured by a titration technique. The relationship between fresh body weight and CO2 production by unfed animals could be represented by the equationM=aW b, M being the CO2 output per individual per day andW the fresh body weight. The constantb, which determines the slope of curve, was 0.808. Respiration of the adult female with 100 mg fresh weight was 1.155±0.250 mg CO2/100 g fresh weight/day or 48.69 mg CO2/g dry weight/day. This value corresponds to 35.81 cal/g fresh weight/day or 150.94 cal/g dry weight/day. Supposing the calorific content of spiders to be 5820 cal/g dry weight, rate of the respiratory energy loss to total energy of the body was estimated to be 2.60%. This rate did not strongly contradict with the loss of fresh body weight before and after the measurement. The metabolic rate showed remarkable fluctuation with changing food supply. The CO2 production of starved individuals decreased to 83.63±16.34% as compared with individuals which were fed before the measurement.

Sperm competition in the melon fly, Bactrocera cucurbitae (Diptera : Tephritidae) : effects of sperm longevity' on sperm precedence

Sperm competition inBactrocera cucurbitae was studied by double matings of one female with normal and sterile males, with different intervals between the first and the second matings and with or without allowing oviposition after the first or the second mating. When the interval was less than 4 days, the last-male sperm precedence,P2, was not different from 0.5, but as the interval was prolonged,P2 was higher than 0.5. There was no significant difference between treatments in which females were allowed to oviposit after the first mating and only after the second mating. The reason for the higherP2 when the interval was long was therefore attributed not to sperm usage for egg fertilization during the two matings but, possibly, to sperm mortality. ThatP2 was 0.5 for shorter intervals suggests that particular sperm replacement mechanisms such as removal and inactivation are absent in B. cucurbitae. Our study is the first to demonstrate a significant effect of short sperm longevity on the last-male sperm precedence.

A field evaluation of the sexual competitiveness of sterile melon flies, Dacus (Zeugodacus) cucurbitae

ABSTRACT. 1 The sexual competitiveness of sterile melon fly males [Ducus (Zeugodacus) cucurbitue Coquillett] was estimated under field conditions. 2 The value of competitiveness was about 0.8 for flies in the fifth to tenth generation from the beginning of mass‐rearing, but decreased to about 0.2 in the eighteenth generation. 3 The sexual competitiveness estimated using laboratory data was near unity even in the final stage of our study. 4 It was concluded that the field evaluation of sexual competitiveness is essential to determine the quality of the insects reared for purposes of pest control.

Life history characteristics and behaviour of the bamboo aphid,Pseudoregma bambucicola (Hemiptera: Pemphigidae), having sterile soldiers

SummaryLife history and behavioural characteristics of the bamboo aphid,Pseudoregma bambucicola (Takahashi), which has sterile soldiers, were studied in the laboratory. The stadium of normal (fertile) first instar larvae was two times longer than that of second instar larvae, and the stadium of soldier-type (sterile) first instar larvae was much longer (max. 116 days) than the stadium of normal first instar, suggesting that soldiers are able to take nutrition from bamboo. Stimulation of larvae with breath, vibration of bamboo shoots or disturbing the larvae with the tip of a fine brush induced significantly more defensive acts by soldiers than those by normal larvae — the latter usually fled. Soldiers did not attack non-kin conspecific intruders or even aphids of different species, suggesting that, in this species, kin-recognition ability is low.

Social behaviour of a subtropical paper wasp, Ropalidia fasciata (F.) : Field observations during founding stage

Field studies showed that the percentage of foundation by multiple foundresses was remarkably high (34.5%) inRopalidia fasciata in Okinawa. Although one of the foundresses of the association usually remained on the nest and others tended to do extranidal activities, the dominance-subordination relation among cofoundresses seemed to be far milder than that reported inPolistes spp. of Europe and North America. Dominance relation sometimes reversed. All the foundresses could receive a share of food and feed larvae. It was suggested that a large risk of parasitism and predation should favour the trait to join foundress associations inR. fasciata.

Why a cicada, Mogannia minuta Matsumura, became a pest of sugarcane: an hypothesis based on the theory of ‘escape’

Abstract. 1. Life tables were constructed for natural populations of a cicada. Mogannia minuta, during an outbreak in sugarcane fields at Okinawa to determine possible reasons for the outbreak.