Life cycle and appearance
Just as with other predatory mites, the following stages of development are followed: egg, larva, protonymph, deutonymph, and adult.
The development of the Amblyseius swirskii population is dependent on the type of food, the availability of food, the temperature and the humidity. A. swirskii, which occurs in the wild in the countries around the Mediterranean Sea, is well suited to warm and humid climate conditions. The critical relative humidity (RH) is approximately 70%.
In a crop, the predatory mites live in the microclimate found in the layer of air right next to the leaf surface. The atmospheric humidity in this layer of air can differ greatly from that in the greenhouse in which the plants are growing.
If the humidity in the microclimate around the leaf surface drops below 70% for a substantial length of time, the eggs of the predatory mite dehydrate and will not hatch. If the RH in the greenhouse drops significantly, the microclimate around the leaf generally ensures an adequate level of humidity. Only in cases of severe necrosis of the leaf due to infestations like spider mite and thrips can the microclimate become a limiting factor. The rate of transpiration decreases in cases of leaf necrosis. Consequently, the leaf temperature is higher and the atmospheric humidity in the layer of air next to the leaf is lower.
The optimum temperature for A. swirskii lies between 25 and 28 degrees Celsius. A. swirskii is better able to survive at high temperatures than A. cucumeris. This has been demonstrated by Spanish research in sweet pepper crops.
The development from egg to the adult phase takes only five to six days at 26°C. If sufficient food is available, A. swirskii lays an average of two eggs per female per day.
The predatory mite does not go into diapause (dormancy) in response to shorter days or lower temperatures. This means that the predatory mite is also active on shorter days (with less than 12 hours of light). The temperature is an important aspect, however: If the temperature falls below 15 degrees Celsius, the predatory mite will be virtually inactive. A. swirskii can survive such a drop in temperature, but is not resistant to frost.
Mites can be identified by the fact that the adults have eight legs and the body is a whole unit, not made up of segments.
Amblyseius swirskii belongs to the Phytoseiidae family. This group of predatory mites is distinctive in that it has relatively few hairs on its back, 20 pairs of hairs at most. The commercially available predatory mites such as Amblyseius cucumeris, Amblyseius degenerans, Amblyseius californicus, and Phytoseiulus persimilis all belong to the Phytoseiidae family. Some are easy to recognise by their colour. For example, Amblyseius degenerans (which is released in sweet pepper crops) is always dark brown or black in colour. The spider mite specialist, Phytoseiulus persimilis is always bright red.
A. swirskii cannot be distinguished from a number of other predatory mites such as A. cucumeris, A. californicus, Amblyseius barkeri or Amblyseius andersoni with the naked eye or with a magnifying glass.
The differences in appearance are subtle and can only be seen under a microscope. However, if you want to be sure which predatory mite you are dealing with, you must seek an expert to determine the species.
The colour is completely dependent on what the mites have been eating. This can vary from dark red, to purple, to light yellow. With thrips and whitefly as prey, the colour tends to be a kind of light orange. It is also difficult to tell from its behaviour which predatory mite you are dealing with.