The influence of initial age structure on predator-prey interaction

Initial age structure influences the growth of a prey population and the outcome of the predator-prey interaction. In order to quantify that influence, we employed a simple numerical model using experimental data from the system Tetranychus urticae-Phytoseiulus persimilis. Four major points were drawn from the results: (1) A population created by young females grows much faster than a population created by the same number of females but distributed among the stable age structure. Final number of individuals after a few generations is then much higher than what a plant could support. Consequently, a stable age structure is probably never achieved under these conditions; (2) In the presence of a predator, such a population can persist for a sufficiently long time to overexploit its host plant and to produce enough individuals to allow dispersal; (3) The impact of the predator on the prey population is drastically different according to its own age structure at the beginning of the interaction; and (4) Predators disturb the prey age structure during the course of interactions and thus maintain the prey growth potential at a high level. These points constitute an important adaptation that determine the persistence of the prey and the predator at a metapopulation level. They bring a new insight on the adaptive characters of young female dispersal. (C) 1999 Published by Elsevier Science B.V. All rights reserved

Une technique d'évaluation de la sensibilité variétale au tétranyque tisserand, Tetranychus urticae Koch (Acari: Tetranychidae). Application au haricot, au concombre, à la tomate et au fraisier

Varietal resistance is a major element in the integrated control of spider mites; it is the least expensive and the easiest technique for plant protection. In this context, the purpose of this study was to develop a rapid and reliable technique for evaluating strain susceptibility to spider mites. In laboratory conditions (24 degrees C), >99% mite increase potential was already expressed on d 12 of the oviposition period. The curve of cumulated fecundity, limited to 12 d, and particularly the size of the F1 generation, constitute useful criteria for determining susceptible and resistant varieties. After fitting a logistic function and comparing asymptotic confidence intervals, a statistical analysis could be performed. In this manner, 17 bean, 6 cucumber, 4 tomato and 6 strawberry varieties were tested. Significant differences appeared, and some strawberry varieties may even be considered as totally 'resistant' to Tetranychus urticae

The missing stase in spider mites (Acari : Tetranychidae) : when the adult is not the imago

Apart from a calyptostatic prelarva, the ontogeny of spider mites is limited to one six-legged immature followed by three eight-legged instars. The usage consists in naming them: larva, protonymph, deutonymph and adult. In this study, the "missing stase" and different associated hypotheses are explored using diverse approaches: light microscopy, electron microscopy, comparative chaetotaxy, ontogenetic trajectories, and interspecific comparisons. It turns out that the imago (new name for the last stase) is missing in Tetranychus urticae and that its post-embryonic develop- ment may be summarized as follows: prelarva (calyptostase), larva (six-legged stase), protonymph (eight-legged stase),deutonymph (eight-legged stase), tritonymph (with paedogenesis, i.e. precocious development of sexual maturity). The respiratory role of the prelarva is emphasized. The current terminology of post-embryonic development is confusing as it pertains to three approaches: the state or form description, the instar standard and the stase paradigm

[The Effect of Fungus Diseases On Integrated Pest-control in Glasshouses]

In glasshouse crops, more and more natural enemies are being introduced to control the development of the major pests. A new biological control strategy in tomato crop is studied in our center which consists of using only predators and parasitoids instead of insecticides and acaricides. But it could be that this reduction of pesticides - only fungicides are still necessary - has some impact on aerial fungi development and thus makes it necessary to increase the number of treatments against these. To test this assumption we have grown tomatoes in glasshouses that were divided into two identical parts so that a classical control by use of acaricides and insecticides is applied in one part and a biological control, only by release of natural auxiliaries in the other. The development in the crop of powdery mildew (Leveillula taurica) and grey mould (Botrytis cinerea) was determined in the two cases in view of the fact that an equal number of treatments against these fungi was applied on both sides. As a result we observe a more extent development of these fungi in the crop where biological pest control is applied. It would appear that insecticides and acaricides have an effect on these fungi and their application would reduce these fungal diseases problems in a crop. In conclusion, it seems necessary to find alternative methods of fungal control; if not, the benefits of fewer pesticides use will be lost

Demographic Parameters of Frankliniella-occidentalis (pergande) (thysanoptera, Thripidae)

In less than 3 years (1986-1988) Frankliniella occidentalis, imported from USA, had spread throughout Europe. It is a polyphagous Thysanoptera which causes important yield loss in greenhouses, in both vegetable and ornamental crops. Ho Never, insufficient data are available on its basic biology. The purpose of this work was thus to establish its life table. A cohort of 95 individuals were observed day during their whole lifetime. The development time is short, 14.2 days, and no significant difference has been observed between males and females. The mean fecundity is high, as 43.17 larvae were produced by a single female. The intrinsic rate of natural increase is 0.140, meaning that the theoretical population may double in 4.94 days

Feedback stabilization of fed-batch bioreactors: Non-monotonic growth kinetics

This paper deals with the design of a feedback controller for fed-batch microbial conversion processes that forces the substrate concentration C-S to a desired setpoint C-S*, starting from an arbitrary (initial) substrate concentration when non-monotonic growth kinetics apply. This problem is representative for a lot of industrial fermentation processes, with the baker's yeast fermentation as a well-known example. It is assumed that the specific growth rate mu is function of the substrate concentration only. A first approach exploits the availability of on-line measurements of both the substrate and biomass concentration. A second approach is merely based on on-line measurements of the biomass concentration, which provide an estimate for the specific growth rate. After a reformulation of the substrate concentration setpoint into a specific growth rate setpoint, it is demonstrated that the fed-batch process can still be stabilized around any desired operating point along the non-monotonic kinetics

Characterizing indices of damage of tomato by the twospotted spider mite, tetranychus urticae Koch (Acari : Tetranychidae) to achieve biological control.

Indices of damage caused by the two-spotted spider mite (Tetranychus urticae) on tomato are described to provide information for biological control using the predator mite Phytoseiulus persimilis. Degrees of a scale of 1-5 for damage to leaflet and leaf, which when integrated to the plant can distinguish fifty classes of damage (0.0-5.0), are characterized by a number of mites and by a population structure. Scores of 3 for leaflet and leaf damage reveal 99-95% adult females and eggs, marking the end of the colonization phase. The further development of foliage damage to a score of 4 corresponds to the activity of new migrating forms (27-21%) emerging from the important egg sub-population. A plant with a damage degree of 2.0 and 2.5 has 14% and 50% respectively of its ten upper leaves presenting a damage degree of 3 and supporting 6,500 and 15,500 spider mites respectively. These scores seem to be the maximum tolerable levels. Leaves damaged at score 4 appear on plants classified at a mean leaf damage index of 2.8 and supporting 23,100 spider mites