Search

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.