Factors influencing the acaricidal activity of flucycloxuron

Contact activities of flucycloxuron on immature stages of the two-spotted spider mite (Tetranychus urticae (Koch)) and the European red mite (Panonychus ulmi (Koch)) gradually decrease in the successive developmental stages. The levels of contact activity of flucycloxuron on larvae and protonymphs ofT. urticae andP. ulmi are of the same order. Deutonymphs ofT. urticae are less susceptible to contact activity than the similar stage ofP. ulmi. In adultT. urticae, the transovarial ovicidal activity was used as an indicator for cuticular penetration. More than 90% of the maximal penetration into adult mites occurs within 8 h. Reversibility of the transovarial activity was not observed after 24 h, but did occur after a subsequent 48 h stay on untreated leaves. The ovo-larvicidal activity of flucycloxuron onP. ulmi after treatment of apple leaves is strongly negatively influenced by leaf age, partly by lower retention of the spray liquid on the leaves. Leaf penetration was measured by application of flucycloxuron on leaf uppersides and assessment of the transovarial activity in mites (P. ulmi orT. urticae) infested on the undersides, one day after treatment. In this test system, leaf penetration was found to be strongly species dependent. Penetration was high in cucumber, moderate in French beans, cotton, roses and strawberry, but low in apple and pepper. Leaf penetration in French bean plants is drastically reduced at increasing leaf age. The overall positive effect of increase in relative air humidity on leaf penetration, is statistically highly significant (P=0.001) for French beans and almost significant (P=0.08) for cucumbers. WithT. urticae on French bean it was found that in this test flucycloxuron needs more than one day for maximal leaf penetration. Although in apple leaves penetration from uppersides was low, penetration from undersides was much higher. The surfactants Arkopal N 130, Silwet L-77 and X2-5309 enhance penetration from leaf under-sides.