Bonding of fibres

The permethrin molecule (M.W. 391), is equal to or smaller than many of dye molecules. Therefore B.Greenwood (pers.comm.) considers that there is no reason why it should not penetrate the fibres and/or adhere to their surface i.e. permethrin can be thought of as a colourless dye. He also points out that nylon is more amorphous, (less crystalline) than Terylene; therefore the spaces will facilitate easier entry of permethrin into the fibres compared with Terylene. But N.Peacock (pers. comm.) considers that the C=0 bond on permethrin might cause attachment to nylon. He also points out that benzene, which is smaller than permethrin, has difficulty in penetrating nylon. Permethrin might therefore have difficulty penetrating nylon and Terylene, as compared to cotton which is much more easily swelled by water (see section on M.R.). A sw elling agent such as form ic acid m ight partially dissolve the fibre structure, allowing entry of the pyrethroids. However, if this were done they might then not be accessible to the insects.

Water swells fibres, and in going from the dry to the wet state, nylon and cotton increase in length by 12% and in diameter by 5 & 14% respectively (Lewin and Pearce, 1985). The amount of swelling that occurs depends on the amount of amorphous material and the presence of polar groups. Silks and nylon, for example, have the same amount of amorphous material but there is a larger number of polar groups in silk than nylon so its swelling is much greater.

Since dyes (and presumably pyrethroids) are absorbed exclusively into the amorphous regions of fibres, their availability to insects will depend on the amount of amorphous region - thus cotton’s relatively poor performance may be due to the sequestration of pyrethroid into inaccessible regions.

In the dyeing (impregnation) of cotton, first the interchain hydrogen bonding in the accessible regions is disrupted by the water molecules of the dye bath and then the larger dye molecules can penetrate. The dye molecules also form hydrogen bonds (by means of their own reactive substituents) to the cellulose molecules. Because of their size, shape and chemical nature, the dye molecules remain in the fibre on drying (Schick, 1977).

M. Wilding (pers. comm.) suggests that the pyrethroid molecule will probably have some affinity for polyester and cotton and less for nylon, because of the two benzene rings that would form Van der Waal bonds to the benzene rings of polyester and the hydrophobic surface of cellulose.

T. Shaw (pers. comm.) suggests that the reason the nylon performs better than cotton is that the emulsion particles of the E.C. are usually stabilised by negative charge and negative charges will exhaust onto nylon better that cotton.

Laveissiere et al. (1987) point out that both the fibre and the weave effect the efficiency o f screens im pregnated with pyrethroids fo r tsetse control. A fter impregnation with deltamethrin or alphacypermethrin synthetic fabrics (polyester, acrylic and especially polyamide) caused the highest mortality. A screen made with closely-woven cotton/polyester fabric made of thin thread allowed good fixation of insecticide but prevented tsetse from picking up a lethal dose.

Table 2.1 (a) shows that in the present studies the number of replicates per fibre/pyrethroid varied from the comm only accepted minimum of 3, up to 12 (permethrin/nylon). If theX ^ test fo r heterogeneity about the regression line was found to be significant after analysing three replicates, more replicates were carried out, and these were continued until the X^ value for all the replicates taken together declined below the critical value for statistical significance.

Cotton required fewer replicates than nylon, except in the case of deltamethrin which required only 3 replicates on both fibres. This may be a clue as to why the toxicity of deltamethrin was similar on both cotton and nylon i.e. deltamethrin may not sink into the cotton fibres as much as do permethrin and lambdacyhalothrin. The retention of these two pyrethroids on the surface of the nylon fibres may increase their toxicity as well as their irritancy. The latter property may well make it more difficult to carry out repeatable tests and hence have led to the need for more replicates (twice as many replicates were needed on nylon as on cotton).

The easier availability of permethrin on the nylon fibres than on cotton can be visualised from Plate 2. The bioassay data show that, in comparing the three fabrics,

nylon recorded the lowest LC^q and LCqq values for all three insecticides, except in

the case of the LC^g value for perm ethrin on Terylene although even here the

confidence lim its overlap. H ossain et al., (1989 [a]) found the LC^q value of

permethrin impregnated cotton nets to be about three times as great as that of permethrin impregnated nylon nets.

As already mentioned, the difference between cotton and nylon w ith deltamethrin was not significant (confidence limits overlap). It is noteworthy that Hervy and Sales (1980) and Wu N eng (cited in Curtis et al, 1989) reported that, (based on bioassays with prolonged exposures), deltamethrin on cotton had a slight advantage over deltamethrin on nylon. Where this insecticide is being used (as in China) and where locally produced cotton nets are in widespread use, there is no case for attempting to switch to the use of nylon nets.

Although lambdacyhalothrin and deltamethrin performed similarly on cotton and T erylene, lam bdacyhalothrin was m arkedly more active on nylon than deltamethrin. Where a malaria control programme introduces the use of bednets impregnated with permethrin or lambdacyhalothrin, the fibre of choice should be nylon.

Terylene (polyester) recorded the highest LC^q and LCqq values of the three

fibres for deltamethrin and lambdacyhalothrin. This may be due to a bonding interaction between the alpha cyano-pyrethroids and Terylene, which may not occur with permethrin (which is not an alpha cyano-compound). Terylene is a specialised, relatively expensive polyester, subjected to many finishes ( e.g. optical brighteners) which may modify its pyrethroid absorption/binding characteristics and affect its performance.

Comparison of the two alpha cyano-pyrethroids with permethrin shows very large differences in efficacy. It is generally considered that deltam ethrin and

lambdacyhalothrin are about 1 0 times more biologically active than permethrin.

However, in the present study lambdacyhalothrin ranged from 28 times more effective

than permethrin at the LC^q level on Terylene to 655 times more effective at the

LC^o level on nylon. Similarly, deltamethrin showed 8 times greater toxicity at the

LCgg level on Terylene and 393 times greater toxicity at the LC^q level on cotton.

Previous comparisons of pyrethroids have generally been on filter paper, mud walls, by fogging, aerosols or topical application. Filter paper and mud may absorb the pyrethroids in a similar manner to that proposed for cotton. This might account for its consistently low performance as compared to nylon. As Whickham et al. (1974) emphasize, the mode of pick-up of insecticide is a cause of variation in the results of different test methods. Chadwick (1985) comments that the literature shows many different test conditions with a corresponding wide range of mortality values so that data are difficult to interpret or reconcile.

A comparison of different surfaces and E.C. and wettable powder (W.P.) formulations shows that surface and formulation both greatly affect uptake and

mortality. For example, with permethrin E.C. (200 mg/m^) male Blattella germanica

take up 2150 ng of permethrin on glass with a corresponding 1 0 0% mortality whereas

on plaster they take up 6 ng causing no mortality (Chadwick 1985). This seems to be

somewhat analogous to the situation of pyrethroid impregnated nylon and cotton fibres.

2.2 Efficacy of pyrethroid treated polypropylene