A systems model and computer simulation of the green rice leafhopper populations in control programmes

A simulation model of the green rice leafhopper-spiders system was presented. The validity of this model for simulation purpose was tested by comparing the calculated values with observed ones (Fig. 4). The effectiveness of various control measures against leafhoppers was evaluated by computer simulations. The computer simulation demonstrated that the wrong use of selective insecticide, contrary to expectation, brought an increase in the pest density, i. e., that the egg densities of leafhoppers in the 2nd and 3rd generations are increased by the insecticidal application in February, while they are decreased by the July application (Table 2). To obtain satisfactory control by sterile-male release, 320,000 sterile-male per square kilometer should be released even in the combined use with insecticides (Fig. 5). The escape of leafhoppers from predation by spiders was demonstrated by the simulation. It is suggested that spiders are able to suppress the leafhopper populations at a low density when there is a very favorable balance between spiders and leafhoppers, and this condition may be realized by sophisticated use of selective insecticides (Fig. 6). Factors and/or processes which have to be involved in a more improved systems model are discussed.     

Population dynamics of a migrant skipper butterflyParnara guttata (Lepidoptera: Hesperiidae) II. Survival rates of immature stages in paddy fields

Population Ecology - Survival rates and mortality factors of a migrant skipperParnara guttata were censused in paddy fields in 4 localities of central and western Japan during 1975–1980, and...     

Estimating the survival rate and mean longevity for adults in a field population of the green rice leafhopper,Nephotettix cincticeps Uhler,by the application ofHokyo andKiritani’s method

Application of the female dissection method proposed byHokyo andKiritani (1967) was attempted in both 1968 and 1969 to estimate the daily survival rate and the mean longevity for the adult population of the green rice leafhopper, Nephotettix cincticeps, in a paddy field. The estimated mean longevity for females was far shorter than the physiological longevity of this species, ranging from 4 to 7 days with some variation between different generations. This could explain the remarkable discontinuity among successive generations which proved to form an important feature of the pattern of seasonal population changes of this insect. Between the two years, the estimated survival rate (and hence the mean longevity also) was negatively correlated to the estimated population size of adults that emerged in either of the two successive generations. This apparent density dependence suggests the possibility that the adult survival, including the effect of dispersal, plays some critical role in bringing about the remarkable population stability from year to year shown by the field population of N. cincticeps.     

The sterile insect release method on species with two-stage life cycles

A continuous-time density dependent model was constructed of a species with a two stage life cycle. This model has a unique stable equilibrium. With the introduction of steriles at constant rate a second positive unstable steady state appears; this condition does not depend on the mode of action within the life cycle of the density dependence or its relative strength. A comparison was made of the effects of having the density dependence in each of larval and adult recruitment and larval and adult losses. It was found that if only adult recruitment is denisty dependent, then adult numbers can actually increase with the release of steriles provided density independent recruitment greatly exceeds density independent losses. Sterile releases were often more effective against larvae than against adults, although in some cases not importantly so. Density dependence in recruitment gives much lower equilibrium values than when density dependence of comparable strength is in the mortality. The release rates needed to cause extinction were generally between 0.1 and 0.5 of the larval equilibrium with no sterile releases except when the density dependence is predominantly in adult recruitment, in which case much higher release rates are required.     

Reproductive properties of the oriental chinch bug,Cavelerius saccharivorus Okajima (Heteroptera: Lygaeidae), in relation to its wing polymorphism

Reproductive properties of the oriental chinch bug, Cavelerius saccharivorus, were investigated for three types of female adults: brachypters produced in both high and low density populations and macropters produced in high density populations. With respect to egg-laying potential in terms of clutch size, clutch number, and total fecundity, macropters were superior to brachypters produced in the same high density population, although inferior to brachypters produced in the low density population. The mean fecundity of female adults in each type was positively correlated with mean body length. Macropterous females markedly prolonged their initiation of both copulation and oviposition and had a much longer life span compared with both types of brachypters. However, there was no fundamental difference in reproductive schedules between the two types of brachypters.     

Evaluation of the role ofMicrovelia douglasi atrolineata (Bergroth) (Heteroptera: Veliidae) as predator of the brown planthopperNilaparvata lugens (Stål) (Homoptera: Delphacidae)

The biology of the veliid bug M. d. atrolineata, its predatory behavior, and the effects of plural hunting were studied to evaluate its role as a predator of the brown planthopper Nilaparvata lugens in the Philippines. The probability of planthoppers falling onto the water surface and provision of habitat continuity was measured by a sticky trap placed at the base of rice hills in a greenhouse and in paddy fields. The developmental period of immature stages combined was 21 days. If given prey, females laid 25 eggs on the average during an adult life span of 18 days. Starved adults could survive for only 3–5 days. The functional response to prey density was sigmoid, and the maximum number of prey killed was 7 per day. Prey feeding was completed in 12–36 min. The percentage of successful prey attacks averaged 5–8%, decreasing with higher (and larger) developmental. stages of prey, but adult prey were found the soonest. Plural hunting increased the probability of capturing prey by as much as 2.5 times that by individual hunting. Late-instar nymphs, which may be more active, fell from rice hills in a greenhouse more than early-instar nymphs, and the number falling increased with density. In the field the percentage of planthoppers falling to the water in 1 day varied considerably, from 1% for nymphs in one field to 67% for adults in another field. On the basis of work described above and given the high density of veliid predators in flooded paddy fields of tropical Asia, M. d. atrolineata is considered one of the most important natural enemies of the brown planthopper.     

Population growth of herbivorous rotifers and their predator ( Asplanchna ) on urban wastewaters

We studied the population growth of three Brachionus species ( B. calyciflorus , B. patulus and B. rubens ) using domestic (Mexico City) wastewater at different phases of treatment (treated: type C, partially treated: type B and crude: type A). We also evaluated growth characteristics of a predatory rotifer ( Asplanchna sieboldi ) fed B. rubens raised on wastewater and compared the growth rates of animals fed cultured algae. All the three tested brachionid species grew well in controls (i.e. fed on the alga Chlorella ). However, populations of B. calyciflorus and B. patulus did not grow on the fully treated wastewater (type C). All three brachionid species grew poorly on par tially treated wastewater (type B). In crude wastewater, B. rubens reached densities as high as 200 ind. ml ^–1 . Regardless of Brachionus species, the rates of population increase per day (r) varied between 0.29 and 0.4 in the controls. B. rubens in crude wastewater experienced steep mortality in the first two days but stabilized thereafter. It had generation times, which varied from 3 to 5 days depending on the treatment. Prey ( B. rubens ) raised on wastewater supported better population growth of A. sieboldi , the highest being from crude wastes. The growth rate of A. sieboldi fed B. rubens , raised on crude wastewater, was the highest ( 0.8 d ^–1 ). Thus, the present study showed that culturing of certain species of brachionid rotifers on crude wastewater or partially treated is feasible without addition of alga.     

Relationship between species richness and spatial and temporal distance from seed source in semi‐natural grassland

Question: How do traditional management practices of field margins maintain the biodiversity of native grassland species? Location: Semi‐natural grassland on the field margins of traditional and consolidated agricultural fields on Awaji Island, central Japan. Methods: The distance to the nearest traditional field margin to the study sites was determined because the traditional field was considered as a seed source of native vegetation to the semi‐natural grasslands under study. We selected field margins in consolidated fields of different ages and distances from seed sources. Indicator species for both field types were sought. Regression analysis and detrended correspondence analysis (DCA) were used to determine the effect of spatial and temporal distances on the species composition of native vegetation. Results: Species richness differed significantly between the margin of traditional and consolidated fields. We identified significant indicator species of traditional fields, but not of consolidated fields. In consolidated fields, species richness increased significantly with age and decreased significantly with increasing distance to the source. At younger sites, species richness decreased faster with distance to the source because of strong negative correlation, but not at older sites. DCA ordination plots similarly indicated that similarities of vegetation composition in consolidated and traditional fields decreased with distance, and the effect of distance decreased with age. Conclusions: The species composition of the grassland margins of consolidated field was more similar to the margins of traditional fields if the consolidated fields were older, and/or closer to traditional fields. This pattern suggests that dispersal may play a role in the establishment of species on field margins.     

The effect of the rate of successful dispersal of a Phytoseiid mite,Phytoseiulus persimilis Athias-Henriot (Acarina: Phytoseiidae) on the persistence in the interactive system between the predator and its prey

Dispersal behaviour was studied on a predacious phytoseiid mite, Phytoseiulus persimilisAthias-Henriot in response to the density of its prey, Tetranychus kanzawaiKishida . And the effect of the change in the rate of successful dispersal of the predators among patches was tested on the persistence of the predator-prey system. The results of the study are summarized as follows:

1. With a severe decline in prey density available per individual predator, the predators exhibited a marked behavioural change and dispersed to other areas.
2. When two neighboring host plants did not touch with each other and the dispersal of the predators was possible only through a 20cm plywood “bridge” which connected the two plants, the rate of successful dispersal of the predators was only 20 to 25%, whereas it was 95% when the plants touched with each other.
3. In the system where 32 host plants touched with each other, the predators succeeded in immigrating into all the plants by the 16th day, and they completely eliminated all the prey by the 33rd day.
4. In another system, the 32 plants were equally divided into eight “patches” which were connected with each other with two of the bridges and the predators could move between them only through the bridges as mentioned above. In such a system the predators and their prey coexisted about three times longer than in the other one.
5. It is suggested that for a longer continued coexistence of the predators and prey, it would be necessary to introduce more physical barrier against the dispersal of the predators and to provide more chances for the prey to disperse.

     

Population behaviour of the southern green stink bug,Nezara viridula,with special reference to the developmental stages of early-planted paddy

Population behaviour of adults and 5th-instar nymphs of Nezara viridula L. was analysed by means of the marking-and-recapture method in an early-planted paddy field. The field contained five varieties of rice which differend in growth states. It was estimated that a total of more, than 7,000 adults of the first generation, in which at least 3,000 were females, invaded the field from early July to middle August. Egg-mass census data, however, indicated that only 10 per cent or less of the females participated in egg-laying. This was largely due to the, low rate of adult survival. The adults preferred younger plants, for both feeding and oviposition. The method described byIwao et al. (1966) permitted estimate that 3,300–3,400 of the 5 th-instar nymphs and 1,100–1,200 of the adults of the second generation were produced from 298 egg-masses (25, 700 eggs); while 95–6 per cent of the individuals were thought to have died before reaching adulthood. Most of the 5 th-instar nymphs moved less than 4 m in three days as long as the condition of food plants remained suitable, but they tended to move more towards younger plants when those on which they lived became too mature. The apparent survival rate of the second generation adults was very low, probably due both to a rapid emigration and a high mortality of newly-emerged adults.