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Laboratory studies were carried out to evaluate the effect of humidity on some biological aspects of T. cucurbitacearum and T. arabicus. Both mite species were reared in humidity-controlled desiccators, and were subject to daily observation to record the sequencial duration of different stages throughout the life cycle. The following results could be reached: Increased relative humidity (to 60%) prolonged the preoviposition period of T. arabicus but had no obvious effect on T. cucurbitacearum. Regardless of mite species, egg incubation period became longer when RH was higher. There was no evidence of humidity effect on all three quiescent stages of any of the tested species. High relative humidity (60%) generally increased the duration of the active larval and nymphal stages of T. arabicus, but did not affect the stages of T. cucurbitacearum. The generation period was longer for T. cucurbitacearum, at both levels of humidity tested. Zusammenfassung Artenunterschiede zwischen Tetranychus cucurbitacearum (Say.) und T. arabicus hinsichtlich der Feuchtigkeitsvalenz Es wurden Laboruntersuchungen uber die Wirkung der Luftfeuchtigkeit auf einige biologische Aspekte der 2 Tetranychus-Arten durchgefuhrt. Die Tiere wurden in luftfeuchtekonstanten Kammern gehalten und taglich hinsichtlich der Entwicklung ihrer Stadien kontrolliert. Ein Anstieg der rel. LF auf 60% verlangerte die Praovipositionsperioden von T. arabicus nicht, wohl aber von T. cucurbitacearum. Bei beiden Milbenarten stieg die Eientwicklungsdauer mit der Luftfeuchte an. Alle 3 Ruhestadien beider Arten zeigten keine deutliche Abhangigkeit von der LF. Hohe LF (60%) verlangerte allgemein die Dauer der aktiven Larven- und Nymphenstadien von T. arabicus, nicht aber von T. cucurbitacearum. Die Generationsdauer war an beiden getesteten LF-Stufen bei T. cucurbitacearum langer als bei T. arabicus.

Acaricidal field experiments were carried out and the results compared with simultaneously conducted laboratory experiments on the same mite strains. Two tetranychid mite species occurring on Egyptian cotton, Tetranychus arabicus Attiah and T. cucurbitacearum Sayed, were investigated. Population counts on the experimental sites over the 1969–70 seasons demonstrated that at the northern sites of Behera (western Delta) and Dakahlia (eastern Delta) the red species T. cucurbitacearum was dominant, whereas in the south, the green species T. arabicus prevailed. Significant differences between treatments in field experiments corresponded to differences in LC50 of ca. 7 × in the laboratory experiments using the slide-dip technique. Under conditions of high resistance towards OP and chlorinated hydrocarbon acaricides, chlordimeform was the most effective acaricide.

It has not been possible to demonstrate an increase in oviposition rate in Tetranychus urticae Koch and T. telarius (L.) owing to DDT treatment. Treatments included direct application, holding on treated bean leaves, rearing on bean plants inserted in water containing DDT or combinations of such treatments. It was not possible to confirm a report that daughters of treated mites produced more eggs than did the parent females. A definite harmful effect is evidenced by early mortality, lowered oviposition rate, and restlessness. The harmful effect is intensified by high temperature.

Resistance to miticides was detected in 2 mite species which are major pests on cotton in Egypt. There were 2 cases of dicofol resistance in the red species around Damanhur and Fayum, which must be called regional. In the green species resistance against amidithion was diagnosed in 2 populations-Kafr El Sheikh and the sprayed area of Giza. Probing of populations from fields near Kafr El Sheikh did not reveal further resistance, so this case must be called focal], contrary to the dicofol resistance of Damanhur and Datum regions. There are no such complete data for the environment of the treated area of Giza, but the values of the Imbaba population (Giza) show that resistance has not yet spread to the farther environment of the sprayed area of Giza. Mite populations of this neighborhood deserve attention, as it is here that the green form demonstrates multiple resistance against organophosphates and organochlorines. Emphasis is placed on the diagnostic pan in plant protection featuring toxicological analysis of field populations of the pests to be controlled. In highly intensive cultures pest control thus may he raised from empirism to discriminative action.

Tetranychus urticae Koeh and T. telarius (L.) exhibit a decided tendency to disperse from crowded colonies. If dense colonies are confined, longevity and oviposition by adults are reduced, and young have less success in maturing. Crowding does not affect hatchability of eggs. Treatment with DDT does not enhance dispersion except at high levels of dosage, and the high levels are toxic, especially to emhryonated eggs and larvae. Emulsified DDT is more toxic than DDT in wettable powder suspension. It is postulated that the outbreaks of spider mites resulting from DDT treatment of crops are the result mainly of: DDT causing dispersion indirectly through the removal of natural enemies, which in turn allows dense local populations to develop. This results in dispersion as a result of crowding, and with dispersion to predator-free surfaces, there follows an increase in the population to economically harmful levels. The terminology used for the species of Tetranychus follows that of Boudreaux and Dosse (1963).

The 2 forms of tetranychid mites occurring on Egyptian cotton are presently designated the red and green forms of the Tetranychus telarius complex. Crossmating experiments with both forms were undertaken to demonstrate their interbreeding potential. These tests revealed intersterility between both forms, so that they can be regarded as separate species. Crosses between local populations from different parts of Egypt did not show reproductive isolation within either red or green species. Both show definite differences in their response to acaricides. The significance of these findings with regard to the gradation of both species in the course of 1 year is discussed.

Among pests of economic importance infesting citrus trees in Egypt are citrus mites. Not only do they affect the yield owing to depletion of nourishment from infested trees, but they also cause injuries to the fruit and hence minimize their market value. Symptoms caused by rust mites, flat mites and bud mites are well known among citrus growers wherever citrus trees are grown. Although leprosis was reported as a result of flat mite infestations in Florida (Knorr et al., 1957) this disease has not yet been recorded in Egypt.

Oil sprays as well as fumigation with hydrocyanic gas have long been used as the common means of control against scale insects on different species of citrus trees in Egypt. Applications are usually carried out late fall and winter. During the last few years some phosphorus compounds applied in summer were used for the same reason. Meanwhile it was noticed that infestation with citrus mites, especial­ly the flat mite Brevipalpus californicus, has recently increased, a matter causing many shipments of citrus fruits to be rejected for export due to the infestation sym­ptoms. Phosphorus compounds have been reported as a cause of an increase in te­tranychid mites and of populations of aphid and white flies. The following experi­ment was undertaken to investigate the effort of some phosphorus compounds, in­cluding those recommended, against citrus scale insects, on the flat mite popula­tion in comparison with certain acaricides known. to be effective against this mite.

Mites of the family Cheyletidae are often found in association with stored food mites. Their potential ability to eradicate a population of tyroglyphid mites is debatable in spite of the remarkably fierce-looking palpi they possess.

Mites are among the most important pests affecting the citrus industry. Not only do they affect the quantity and the size of produced fruits, but they also reduce their quality due to the symptoms they cause. The citrus flat mite Brevipalpus californicus is one of the most injurious mites infesting all citrus species cultivated in both Lower and Upper Egypt. Studying the biology of that mite is no doubt an essential step towards achieving successful control.

Attiah (1969) recorded the presence of Tetranychus neocalidonicus on citrus trees in a grove at Abo Homnos (Beheirah Governerate) in Egypt. The mites were found more often on ripe fruits than on green ones causing yellowish blotches on the surface of orange and mandarin fruits.

Informations are given that the Green Egyptian Mite, Tetranychus arabicus Attiah is quite distinct from T. urticae Koch, contrary to the meaning of Zaher et al. (1981). Zusammenfassung Zum Status von Tetranychus arabicus Attiah und T. urticae Koch Der systematische Status der beiden Tetranychus-Arten wird erortert. Beide Arten zeigen deutliche Unterschiede und haben entgegen der Meinung von Zaher et al. (1981) als eigene Arten zu gelten.