The population density of parasites, also called «cumulative activity», is assessed taking into account weather conditions for the past 6 weeks.
This cumulative activity is an average over the past 6 weeks, but with an ascending order of importance on these weeks. This combination allows us to see whether the arthropod parasite of interest has had a favourable climate. The more favourable conditions are, the more rapid the parasite life cycle will be and the higher their density will become.
The density index expresses the cumulative activity of arthropods over a determined period of the previous 6 weeks. It is used to construct the density maps with 5 levels of density, ranked from 0 to 100: (0-20), (20-40), (40-60), (60-80), and (80-100), from the less to the most favourable.
Density index calculation
The calculation of cumulative activity is slightly more complex. The density values are calculated according to the weekly fields for the past 6 weeks. Each weekly value is calculated as for the next week. To accumulate the previous weeks, we take into account the exponential aspect of growth related to arthropod incubation over a long period of time.
This accumulation allows forecasting of the biological cycle potential and therefore the variation of the relative importance of the parasite populations.
The following formula can be used to evaluate cumulative activity:
Density = K × log (W1 × W2 × W3 × W4 × W5 × W6)
(with Wi the activity estimated for the previous week i)
The K factor allowing to readjust the logarithmic scale to a 0-1000 window. It is calculated using the following formula :
|k = ||desired max value of accumulation|| = ||1000|
|log (max value of one week) × ∑weeks weight||log (100) × 6|