No abstract

Mites of the family Phytoseiidae, a diverse and widespread group, occur throughout the world from the Arctic through the tropics. The number of described species has increased from fewer than 20 in 1951 (Nesbitt, 1951) to over 450 in 1965 (Chant, personal communication). Opinion differs among' taxon

Experimental studies on the role of dispersion and level of food supply in predator-prey relations were continued. They were conducted under controlled laboratory condi tions using the phytophagous mite Eotetranychus sexma* culatus as prey, and the mite Typhlodromus occidentales as predator.Results give additional support to earlier findings that more stable control of a population results when the en vironment is spatially more heterogeneous and there is a balance between the extent of heterogeneity and the amount of basic food. An excess of plant food at first generated large numbers of prey, and then an excess of predators which overexploited their prey and then starved.By using a complex spatial environment consisting of three grid-wire shelves in a cabinet, and a wide dispersion of the basic food material, the predator-prey interaction was continued for 490 days, and the end was brought about by a disturbing disease rather than by overexploitation on the part of the predatory mite.In contrast, trebling the quantity of basic food while maintaining other factors unchanged apparently created an imbalance between numbers of predators generated in the system and the hazards they faced in covering the areas to be searched-thus, they overexploited in each case at the end of the first prey population crash phase.These result^ also suggest the potential role of prédation as a means of synchronizing cycle phase in some natural populations.

The literature pertalOlOg to the bionomics of some of the mos t important tetranychids is r~viewed. Considerable attention is paid to the possihiliries of spider mites reaching pest le~els /as a 'resu lt of environmental factors. It is concluded that more infor--m ation is needed on economic injury levels of various species on d ifferent crops and under various climatic conditions. The in-I fluences of cultural practices and their interactions as related to population development of tetranychids also need more i tu dy. Furthermore, work is need~d on the influence of climatic conditions on development of retranychid mite populations together with possible predictability 'of outbreak situations. Finally; we need to know more about ·th)e fundamental relations between the host-planr-qualiry and spider mite development, and the, impact of spider mites on the development .of their .host p lants.

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