Relationship between T. urticae numbers in samples and the proportion of infested rose leaflets in Colombian rose crops

Abstract: In Colombia there are almost 7000 hectares of flower production and the floriculture sector generates employment for over 150,000 families. In this area of the economy roses are the main flowers produced and exported. However, production is limited by the attack of pests among which the two-spotted spider mite Tetranychus urticae Koch stands out. According to recent information, 30-50% of the budget destined to pesticides in Colombian rose crops are acaricides. To reduce the use of these acaricides it is important to improve the monitoring plans for this mite using methodologies that allow reducing the invested time without significantly diminishing the reliability of the estimates. This is possible if a specific relationship between density and spatial distribution of the population can be established. In this way, the mean population density could be estimated from the proportion of sampling units occupied by the mites. This relationship had been established for many authors in different countries and for different crops but it had never been tested in the Colombian rose production system. For this reason, a mathematical model was evaluated to find the relationship using data from T. urticae samplings on a commercial rose crop. During 18 weeks, T. urticae were counted on leaves of the lower stratum of rose plants corresponding to 56 squares (each square has 50 plants). Mites were counted on five to seven leaflets of three leaves for each square. It was found that the mite population had an aggregated distribution and the mathematical model tested can describe a relationship between the numbers counted and the proportion of sampling units occupied by the mites. More studies are needed to develop this relationship in order to help farmers to formulate an adequate monitoring plan.