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

with Fo, Pe and Pr being local ‘densities’ of foliage, pests and predators expressed
per tree for the sake of simplicity, and γ 3 and γ 3Pe being the migration terms. As
far as parameters are concerned, α i is an intrinsic term of population increase or
decrease, depending on the sign of α i, β ii a term of self-limitation, and β ij a term
of positive or negative interaction between populations.
Fruit mass and sucrose concentration were the fruit quality traits considered in
the simulations. The growth rate of the fruit crop per tree was very roughly described
as depending linearly on the tree leaf area Fo, thus the mean individual growth
rate was
dF
dt
= δ
Fo
n
Modelling Fruit Quality 75
with F being the fruit mass, n the number of fruit per tree and � a parameter.
Sucrose concentration in the fruit was calculated using the SUGAR model presented
previously in this chapter.
We modified the basic model to account for a possible chemical control, in a
way close to the IFP guidelines. The resulting Catiote model made it possible to
trigger insecticide spraying when pest density exceeded a given threshold y (reasoned
pest control). Input values described the efficiency of the product for the possible
targets, pests and predators, and its persistence through the pattern of decreasing
efficiency over time. This decrease is assumed to be linear with time.
Simulations were performed for a period of 110 days ended by fruit harvest, using
a daily time step. Three cases were investigated where the source of pest control
was (i) the plant environment only, (ii) the integrated chemical control only, (iii)
a mixture of both.
6.2. Effect of pests, predators and insecticide spraying on fruit quality and
6.2. environment
When the only source of pest control was the plant environment and when predators
were pest-independent, predators and leaf area slowly increased, while pests
slowly decreased after a few peaks. Final mean fruit mass was about 70 g and sucrose
concentration about 5%. When pest-dependent, predators caused a stronger regulation
of the system. The final mean fruit mass and sucrose concentration, about
100 g and 6% respectively, were higher than in the previous case (Figure 12).
Under reasoned chemical pest control, without any predator influence, the developmental
trends of pests and leaf area levelled off rapidly but permanent oscillations
were encountered. As soon as the effect of insecticide application was over, pests
increased, resulting in a new application and so on. The resulting outputs were a
mean individual fruit mass of 140 g, a sucrose concentration of 7%, and a number
of seven chemical applications.
Combined environmental and reasoned chemical pest control also rapidly yielded
flat and oscillating evolution trends of pests and leaf area (Figure 12). The patterns
were very similar to the previous case for pest-dependent predators. With pestindependent
predators, the oscillations were less frequent. In this situation of double