Detection to ensure taking the appropriate measures at the right time (FR-BE)

Prevention is one of the golden rules of a natural approach to gardening to ward off biological attackers in the garden.  

Detecting their presence as early on as possible allows you to act as soon as possible to avoid an infestation on the whole crop and above all, it allows you to take the appropriate measures at the right time. 
All bugs that are visible to the naked eye and active in full daylight are easy to detect: you just need to go out into your garden regularly and spend some time observing them. But when the critter concerned is only a few millimetres large or is only active at night, it becomes much more difficult, if not impossible to do this: "when the worm is in the fruit," it's already too late!
So the difficulty is determining the exact right time to take action.  Pheromone traps and chromatic traps (yellow sticky traps for example) are monitoring or surveillance tools that help you to decide whether or not to intervene with (preferably) biocontrol products.  

An example is the pheromone traps used against box tree moths, pine processionary caterpillars, white flies and codling moths, amongst other invaders

These pheromone traps can help to ward off codling moths. 
The creature concerned is a small moth 19mm large, active at night. The pheromone trap releases a sexual attractant or "pheromone" naturally emitted by the female butterfly to attract the male at night, so as to be fertilised.  The moths then become trapped on a sticky plate placed in the trap. 
The trap is hung up in the apple tree, it must be checked once a week, with the number of trapped moths noted in the booklet attached to the trap. 
From five catches in two weeks, the apple harvest is threatened, and to avoid this, it becomes necessary to treat the trees with Bacillus thuringiensis (a natural caterpillar insecticide) 10 days later: the period during which the young caterpillars hatch and emerge in the tree, starting their "walking” stage, 2 to 5 days after which they penetrate the fruit and start causing damage. The control window is therefore very short. 

This necessitates the use of surveillance or monitoring tools to avoid blind treatment, unnecessarily.

This favourable treatment period is virtually impossible to determine without the help of a trap, as the moth is nocturnal.