Effects of Gibberellin on Nutrition of the Mites, Tetranychus telarius and Panonychus ulmi

Abstract

Dwarf Horticultural beans and Delta Pine cotton grown in artificial culture in the greenhouse were treated with gibberellin at the rates of 1, 2 and 3 micrograms per plant. Close apple trees grown in semi-field conditions were sprayed with an emulsifiable concentrate of gibberellin at 7- to 10-day intervals during the season at the rates of 10, 50, 100, 250 and 500 p.p.m. in the spray solution. Teranychus telarius (L.) were cultured on detached leaves from all three host plants, and Panonychus ulmi (Koch) were cultured on leaves from the apple trees. Gibberellin treatments increased plant height in beans and cotton and total branch length in apple trees and resulted in reduced populations of T. telarius in beans and apple trees. Effects of gibberellin on mites cultured on cotton leaves were variable. P. ulmi populations were increased by 10 p.p.m. treatment on apple trees but were reduced as were T. telarius on the higher treatments and were correlated positively and significantly with per cent total sugars in the apple leaves. Both species were generally positively related to per cent nitrogen, which was decreased in the apple leaves by gibberellin treatments. Reducing sugar was not significantly correlated with populations of either mite species. It is suggested that there is an optimum level of reducing sugar for mite development and the percentages in these host plants fell below the level.