Phytoseiidae

Although 22 species have been reported, only three species are regularly encountered in French wine-growing regions and have a real functional role in regulating outbreaks:

- Typhlodromus pyri , in all wine-growing regions, except in the PACA and Languedoc-Roussillon regions, where it is present but with less importance,

- Kampimodromus aberrans predominant , a species in the PACA and Languedoc-Roussillon regions, almost non-existent elsewhere,

- Phytoseius finitimus predominant , a species in Corsica.

A 4 ^th species Amblyseius andersoni is quite common in the west of France and Neoseiulus californicus and to a lesser extent Typhlodromus ( Anthoseius ) exhilaratus can meet in the South.

• Biology

In all known species in temperate climates, overwintering occurs in the adult female stage, which was fertilized in the fall. It takes place under the bark or in crevices, under dead leaves. The animals are in a state of diapause, immobile and not feeding.

Phytoseiidae have four discernible developmental stages before the adult stage. The egg is oval and stuck by an adhesive substance to its support (top of the leaf hairs, webs of spider mites, cavities at the intersection of the main and secondary veins, on the blade, etc.). The egg hatches after a few days and the hexapod larva which emerges from it feeds or not depending on the species. Then follow 2 stages called nymphal octopods, which can be distinguished by the size and coloring of the integument which increase. The larva and nymphs change stage after a molt that lasts a short time and is not discernible. In very favorable conditions, development lasts a little more than 3 days to reach 4 weeks in unfavorable conditions. This duration is also variable according to the species. Development time is distributed roughly evenly between embryo development and immature development. High relative humidity (70-80% or more) is favorable for the development of eggs and immatures, but saturated humidity and immersion are usually fatal to eggs. The minimum and maximum temperatures compatible with the development are located in our climates between 10 and 30 ° C with an optimum at 25 ° C. At equal temperature, many phytoseiids grow faster than spider mites.

With the exception of Phytoseiulus persimilis and a few other rare specific species, most species of Phytoseiidae are generalist polyphages. The great enthusiasm for the Phytoseiidae as auxiliaries comes mainly from these few rare "specific" species of spider mites (of the genus Tetranychus ). Unfortunately, in many cases, we are witnessing a confusion between the characteristics of "specific" predators and those of "generalist" predators which are however the overwhelming majority of Phytoseiidae. The latter are very common in more stable, less disturbed agrosystems, with spontaneous woody plants and fruit crops. One hardly ever finds "specific" in this type of environment. Most species of the genera Euseius , Amblyseius , Typhlodromus , Neoseiulus , Kampimodromus , Phytoseius , etc. belong to this group of "generalists". These have a low reproductive potential, compared to "specific". They also have more stable populations, disperse little, being little linked to the need to search for prey because they consume other foods (ie small insects, pollen).

Mating is necessary for egg-laying to begin and several matings are usually observed. The period of sexual maturation often lasts only a few days and the laying takes place for 15 to 30 days. The total length of adult life is variable. The daily laying rate fluctuates, depending on the species and the microclimatic and trophic conditions, from 0.1 to 4.5 eggs but rarely exceeds 3 eggs. It is most often around one egg per female per day. Total fertility is also variable, generally between 50 and 90 eggs / female. The average sex ratio is usually 66% females (but this figure is also variable).

• Food

Pollen seems to be highly prized by many "generalist" species and very favorable to the expression of a significant multiplication potential, which is not the case for "specific" species. This is the case for the two main species of the grapevine, in particular Kampimodromus aberrans . Although qualified above all as predators of spider mites, many species of phytoseiids consume large numbers of eriophyids, tenuipalpids, tarsonemids and even tideids. Certain species develop moreover better on eriophyids than on spider mites and are sometimes stimulated in the predation of the latter by the presence of the former. While some phytoseiides adapt to the weaving of weaving spider mites, such as Phytoseiulus or certain Neoseiulus , N. californicus for example, others such as T. pyri and K. aberrans are hampered and cannot move there . The honeydew of the hemiptera, associated with spider mites or pollen, would allow more important laying. Some species attack eggs or young larvae of hemiptera (aphids, cochineals, whiteflies, psyllids) or thrips (in particular T. pyri ) and a phytophagous diet of certain species has been observed (but without damage). However, it is the spider mites that remain the most consumed prey and, among them, the eggs and the immature stages are the most sought after. The amounts consumed vary greatly depending on the age of the predator, the species and the microclimatic conditions.

• Relationship between predators and prey

The effectiveness of a predator can therefore be defined by an optimal total response, corresponding to a certain density of prey, the predator being moreover capable of adapting its density to that of its prey. Typhlodromus pyri, K. aberrans, T. exhilaratus and P. finitimus can be described as "low density" predator, Amblyseius andersoni and Neoseiulus californicus as "low to medium density" predators.

Some authors have proposed a classification based on the decreasing specificity of predators and the increasing link to the support plant. We distinguish :

- type 1: specific predators of the genus Tetranychus . Four species of the genus Phytoseiulus with the strongest: population growth rate, rapidity of development, fertility, voracity and size.

- type 2: selective predators of spider mites, in particular weavers. Species of the genera Galendromus and Neoseiulus and some species of the genus Typhlodromus ( Anthoseius ). They generally have strengths: rate of increase in numbers, rapidity of development, fertility. Their size and voracity are smaller. Some species consume prey other than spider mites, in particular eriophyids for Neoseiulus . Breeding on pollen is possible.

- type 3: generalist predators. Species of this group are found in all known genera. Species of the genus Tetranychus are among the possible preys but are often more unfavorable than species of other genera which weave less. Eriophyids are potential favorable prey. Tideids can be consumed by Typhlodromus pyri and some species of Paraseiulus . Many insects can also be eaten, including thrips, whiteflies, aphid and mealybug larvae, etc. Type 3 phytoseiids also feed on pollen, plant exudates and some of the plant tissues. Population growth rates are low to medium. Relationships with the host plant are often very important.

- type 4: generalist predators, specific pollen consumers . They are also polyphagous species, but for many species of the genus Euseius , the rate of population growth is often higher with pollen than with prey. Some species consume spider mites but never alone and some others eat eriophyids or thrips and / or whiteflies.

• Ecology and link to the host plant in generalist polyphages

For generalist polyphagous Phytoseiidae, the presence, development and abundance on a plant are therefore generally not totally correlated with those of prey. Most of the Phytoseiidae have a wide range of host plants, but in a given environment only a few plants can be colonized. The factors making these plants favorable or unfavorable are not fully understood. On the leaves, phytoseiides are most often found near the veins, especially in the axils of the ramifications. They hide their spawning there, take shelter, mate there and watch for their prey, undoubtedly finding favorable thermal and hygrometric conditions and fleeing larger predators. The important hair leaves of various plants would allow access to these small species such as K . aberrans or T. pyri , but not that of larger, more competitive species like most Amblyseius and Euseius species . This hairiness also seems to retain more pollen, which these two predators consume in quantity and which allows important reproduction in spring on many host plants, even when the prey is absent. The leaves, home of microscopic mites, are very different according to the plants and have various structures (veins, hairiness, curled borders, domaties, glands, organic compounds, etc.) which give the leaf a complex architecture, which can interfere with or facilitate access, progression, feeding or mating. Thus, we note that certain grape varieties, because of their different leaf characteristics, are more or less favorable to Phytoseiidae.

Despite their poor movement capacities, phytoseiids often disperse fairly quickly within crops, following in particular rows of plants, trellising lines, etc. Their dispersal over long distances is, however, rather passively through the wind or transport by other animals, for example aphids, or by dead leaves. Many species complete their entire cycle on the same host plant. "Generalist" phytoseiids have little tendency to migrate, compared to specialist species which stalk their prey and disperse quickly.