Search behaviour and distribution
For a predatory mite, a plant is a gigantic living environment. If sufficient food is available, such as pollen, thrips larvae/eggs and whitefly larvae, the predatory will tend not to leave the plant. A population of Amblyseius swirskii can grow very quickly if there is enough food. Research by WUR Greenhouse Horticulture in cucumber crops showed that if thrips was present, a population of A. swirskii grew from 10 to on average 600 predatory mites per plant over a period of three weeks.
If the density of predatory mites grows, a shortage of food will be reached more quickly. In that case, the female predatory mites in particular go elsewhere, looking for new plants or parts of the same plant in order to lay eggs and begin a new colony.
In sweet pepper and aubergine crops, A. swirskii is mainly found in the upper levels of the crop. In the other crops that have been researched to date, A. swirskii is found throughout the plant (both at lower and higher levels of the crop). An experiment in cucumber crops showed that the distribution through the plant is very even (see figure 1).
Figure 1 Distribution of A. swirskii in a cucumber plant affected by thrips.
Predatory mites cannot fly and their mobility is therefore limited. For A. swirskii, dispersal generally takes place within a crop row rather than between rows. The predatory mites can disperse themselves via leaves that are connected to each other, or via wires in the crop. Trials carried out by WUR Greenhouse Horticulture showed that predatory mites moved 10m from a ricinus plant into a sweet pepper crop in three weeks. The predatory mites can therefore move around a crop row reasonably quickly. In spite of this, the recommendation is to distribute the mites well when releasing them.
Some parts of the plants themselves appear to be avoided, whereas mites can be found much more readily in certain other spots. It appears that mites avoid greasy spots caused by honeydew from whitefly. A. swirskii is also generally not found in patches of more severe spider mite infestation where the spider mites have spun webs. It is however noteworthy that A. swirskii is often found in patches of rose powdery mildew.