A method for monitoring biocontrol success of whiteflies in green-house tomato crops

Abstract: The availability of chemicals as back-ups of whitefly biocontrol in greenhouse crops
in Finland has reached zero. Additionally, biological control in winter months in artificially
lighted crops is demanding due to lower performance of natural enemies. To address these
challenges, a monitoring method of the whitefly Trialeurodes vaporariorum and its predator
Macrolophus pygmaeus was developed based on long-term studies of the population
development of both species in eight tomato greenhouses with cherry, mini-plum or standard
round tomatoes. The monitoring was coupled with determining the threshold population size of the pest resulting in sticky tomatoes due to excreted honeydew. A proxy for population size of both insect species is the average number of specimens in yellow sticky traps at a density
of 1 trap per 250 m2 inspected weekly. Based on trap counts of both insect species over a year
in a 3250 m2 cherry tomato greenhouse and monitoring the appearance of sticky tomatoes in
whitefly hotspots, a preliminary formula was first developed to assess whether biocontrol was
working successfully or not. The formula and the risk of sticky tomatoes occurring were then
tested and refined in seven other greenhouses over four years. The main factors in the formula came to be the relationship between cumulative whitefly and predator counts over seven successive weeks preceding the current week’s counts, spatial absence of either one of the insects in traps, and the whitefly density per trap in the current week. Eventually, the use of the formula was standardized for an area of 2500-3000 m2 . The monitoring method has been used successfully on several hectares of year-round tomatoes for four years now, with “traffic light” indicators (indicating the risk of sticky tomatoes developing either immediately or in the short term or no risk) informing growers about the status of whitefly control in their crops.