Life cycle, population dynamics

• The wintering of Panonychus ulmi is done in the state of eggs, called "winter eggs", deposited on the base of the branches around the buds or on the old wood of the vine. These eggs laid , fertilized in the fall, without special protection, are able to withstand very negative temperatures (diapause). Note that the harsh winters reduce mortality from abortions and allow a clustering of outbreaks in the spring. These outbreaks are spread over the period of bud break of the vine from 15 ° C of average temperature, but they can extend over about sixty days if the winters are mild.

• After hatching , the mite goes through 7 successive stages in 2 phases: a larval phase with 3 mobile stages called nymphal, interspersed by 3 immobile stages of moult called chrysalis , then an phase adult . The females lay eggs called "summer eggs" on the leaves, for about 15 days. The "winter eggs" will be deposited from August.

• The cycle of this mite is a function of temperature and humidity; it is spread over a minimum of 7 days, for temperatures of 23 to 25 ° C and humidity of 60%. This speed of development quickly allows an overlap of the generations at the end of spring and the presence on the vine of all the stages of development from egg to adult. In the end, there are 4 to 8 generations of spider mites over the year, the number varying according to the geographical area (higher number in the southern zone, lower in the northern zone). It is most frequent in the northern areas and on the Atlantic coast.

• Periods of strong parasitic pressure are observed mainly in July and August, at the end of vegetative growth. The quantities of individuals observed can greatly exceed a hundred per leaf. These conditions of pullulation are encountered:
  • in the absence of predatory mites Typhlodromes;
  • in the case of insecticide-acaricide resistance phenomena;
  • in the case of strong vines vigor.

Note : this mite can be observed on both the underside and the upper side of leaves.