Influence of dislodgeable and penetrated residues on volatilization, codistillation and persistence of mexacarbate in balsam fir foliage
Five spray mixes of mexacarbate (4-dimethylamino-3,5-xylyl N-methylcarbamate) were prepared using different inert ingredients. Among the mixes, ZE-DIL was oil-based, ZE-EML-1 14 and ZE-EML-3409 were emulsions, ZE-WP-W was a wettable powder in water, and ZE-FL-W was a flowable in water. The mixes were sprayed over potted balsam fir [Abies balsamea (L.) Mill.] seedlings in a laboratory chamber and foliar residues were analyzed by gas chromatography. The results showed that the ratios of the initial dislodgeable to penetrated residues were low for ZE-DIL, ZE-EML-114 and ZE-EML-3409, but were high for ZE-WP-W and ZE-FL-W. The latter two spray mixes contained mexacarbate in the particulate form, and contributed to greater surface residues than those of the former three spray mixes which contained mexacarbate in the solution or emulsion form. Foliar residues declined rapidly during 0 to 3 d post-spray, with half-lives (DT 50 ) ranging from 0.84 to 1.73 d. The rate of decline was slow from 3 to 20 d post-spray and the DT 50 values ranged from 2.49 to 4.65 d. Mexacarbate volatilized and codistilled appreciably within 24 h. Volatilization occurred more from dislodgeable residues than from penetrated ones, whereas codistillation occurred from both dislodgeable and penetrated residues. The penetrated residues of ZE-WP-W and ZE-FL-W volatilized less than those of ZE-DIL, ZE-EML-114 and ZE-EML-3409. This behaviour was due to the presence of mexacarbate in the particulate form in the former two spray mixes, as opposed to its presence as a solution or as an emulsion in the latter three spray mixes.