Production Practices and Quality Assessment of Food Crops. Vol. 1

74 M. Génard and F. Lescourret
– response of the plant to pest attacks and diseases, and consequences on fruit
quality;
– regulation of pests in response to the plant, considered as a source of food, and
to pest predators and diseases;
– effect of the plant environment as a source of pests, diseases and pest predators;
– response of the whole system to cultural practices, especially pest and disease
control.
Our purpose was to investigate the possibility of discussing the above-cited points
in an integrated approach. Therefore, we chose the case of pests and a simple and
speculative modelling framework basically a predator-prey model (called ‘Catiote’
and presented in Lescourret et al., 2002). We investigated the effect of a few different
crop protection strategies on the behaviour of a simplified orchard system,
focusing on basic indicators of fruit quality (fruit mass and sugar content) and
environmental quality of the management (number of chemical treatments) that
are concerned in the IFP context.
6.1. Catiote: a predator-prey model under the control of insecticide spraying
Our simplified system is composed of trees in an orchard (foliage and fruit parts),
pests acting on the tree by reducing the leaf area, and their predators. At this stage,
pests (and, correspondingly, predators) can belong to a given species or a guild
(i.e. guild of foliage eaters). To describe the evolution of fruit growth over time
within the foliage/pests/predators subsystem, we used a N-species Lokta-Volterra
model (Lebreton and Millier, 1982). To account for a possible effect of plant
environment in a broad sense, we added a flow of predators to the fruit trees. It is
a rough way to assume that plant reservoirs may have a positive role on the ecological
balance (Rieux et al., 1999). We considered two cases:
– the predator migration flow is constant and the predators do not reproduce because
of the pests they consume. In this case the response of predators to pests is
functional, not numerical. This case will be called ‘pest-independent’;
– the predator migration flow depends on the local density of pests in a simple way,
e.g. linearly, and the reproduction of predators depends on the pests consumed.
This case will be called ‘pest-dependent’.
The equations of the model were
dFo
dt = Fo(α 1 – β 11Fo – β 12Pe)
dPe
dt = Pe(β 21Fo – β 22Pe – β 23Pr)
dPr
dt = Pr(–α 3) + γ 3 (pest independent)
dPr
dt = Pr(–α 3 + β 31Pe) + γ 3Pe (pest dependent)