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Comparisons with standard susceptible insects showed that a strain of Tribolium castaneum, with a specific resistance to malathion and its carboxylic ester analogues, had no cross-resistance to topical applications of natural pyrethrins. Another strain of T. castaneum, showing resistance to many organophosphorus (OP) insecticides, was cross-resistant to pyrethrins ( × 34) and eight synthetic pyrethroids also applied topically; least cross-resistance occurred with resmethrin ( × 2.2), bioresmethrin ( × 3.3) and phenothrin ( × 4.0). Generally larger resistance factors were recorded with formulations synergised by piperonyl butoxide (PB). The greatest cross-resistance encountered was with unsynergised tetramethrin ( × 338). Apart from tetramethrin, factors of synergism did not exceed 5.7 with either the susceptible or multi-OP resistant strains. PB antagonised six of the nine pyrethroids against the multi-OP resistant strain. Antagonism also occurred with two of these six, permethrin (cis: trans ratio 1:3) and 5-prop-2-ynylfurfuryl ( 1RS)-cis,trans-chrysanthemate (‘Prothrin’), against the susceptible strain. Considering only formulations without the synergist, the most effective compounds against the susceptible strain, relative to pyrethrins, were bioresmethrin (2.7) and permethrin (2.4). Similarly with the multi-OP resistant strain the most effective compounds were bioresmethrin (28), resmethrin (14) and permethrin (6.6). Thus the LD50 (the dose required to kill 50% of the test species) for bioresmethrin against the resistant strain (0.14 μg) only slightly exceeded the LD50 for pyrethrins against the susceptible strain (0.12 μg).

A survey in SE England showed that seven out of 17 populations of glasshouse whitefly were resistant to DDT. Resistance to malathion occurred in 15 populations and to resmethrin in eight. Mecarbam and methiocarb gave good kills in screening tests but some insect growth regulators gave variable results. The synthetic pyrethroid permethrin was slightly more effective against a population of whitefly resistant to both DDT and malathion than against a susceptible population. Parthenogenetic females from a population susceptible to DDT and malathion produced almost entirely male progeny. An unsuccessful attempt was made to lower the resistance level of a DDT/malathion resistant strain by introducing these males to resistant females.

Houseflies (Musca domestica) on Danish farms have developed high multiresistance to organophosphorus compounds, after successive use of several OPs, mainly dimethoate, in recent years. Topical application tests 1971–73 with flies from many farms showed that the high OP-resistance did not involve resistance to pyrethroids (± the synergist piperonyl butoxide (pb)) above a level of 3–7 x, unless field pressure with synergised pyrethrum (py/pb) or other pyrethroids was applied. In 1971–72 moderate to high, often heterogeneous, pyrethroid resistance was found on a few trial farms treated frequently with pyrethroid aerosols (mainly py/pb) and in 1973 on most of 23 trial farms treated intensively with aerosols (or space spray) containing py/pb, bioresmethrin ± pb, tetramethrin/pb or tetramethrin/resmethrin. The effect of field pressure with these different pyrethroids on development of pyrethroid resistance is summarised and discussed. Maximum resistance ratios, R/S at LD50-LD95, were: py/pb (1:5), 40->100; bioresmethrin, 191–770; bioresmethrin/pb (1:5), 55–133; tetramethrin/pb (1:5), 171->200; tetramethrin/resmethrin (1:5), 78->370. The intensity of selection pressure with pyrethroids is believed to be an important factor. Although py/pb has been widely used as a supplementary fly control on Danish non-trial farms, pyrethroid resistance has only been found on a few of them.

A sample of houseflies initially collected from a pig farm and found to be resistant to bendiocarb, DDT, gamma-HCH, pyrethrins + piperonyl butoxide (PB), tetrachlorvinphos and trichlorfon, was tested for resistance to knockdown by other toxicants. At the KD50 response level, resistance factors were obtained for: permethrin (× 141), deltamethrin (×96), bioresmethrin + PB (×37), resmethrin + PB (×33), fenitrothion (×94), bromophos (×58), iodofenphos (×42), pirimiphos-methyl (×30), dichlorvos (×22), dimethoate (×9), diazinon (×8), methomyl (×4) and methomyl + PB (×4). The slopes of the dose–response lines were lower for the farm strain than for a susceptible strain. This resulted in an increase of resistance factors at the KD95 level by an average of × 1.6. The houseflies on the farm could not be controlled using space sprays of pyrethrins + PB, although resistance to this toxicant was only ×12. However, control was achieved with a methomyl bait.

Permethrin, cypermethrin, and resmethrin were tested under field conditions as repellents to protect cattle from black flies (Simulium spp.). The chemicals were applied topically to the entire body surface of steers. Ethanolic solutions of technical permethrin, at doses of 1, 2, 4 and 6 mg a. i. kg−1 of body weight, effectively repelled black flies by preventing at least 70% of the flies present from taking a blood meal for up to 8 days, and for at least 11 days at a dose of 12 mg a. i. kg−1. Aqueous mixtures of a 20% permethrin emulsifiable concentrate (e. c.), at doses of 1, 2 and 6 mg a. i. kg−1, effectivelyrepelled black flies for 2, 10 and 11 days, respectively. Aready-to-use 5% permethrin dust, at doses of 1, 2, and 4 mg a. i. kg−1, effectively repelled black flies for 4, 5 and 8 days, respectively. Ethanolic solutions of technical cypermethrin, at doses of 1 and 2 mg a. i. kg−1, repelled black flies for 3 and 4 days, respectively. Aqueous mixtures of a 40% cypermethrin e. c., at doses of 2 and 4 mg a. i. kg−1, repelled black flies for at least 5 days. Ethanolic solutions of technical resmethrin, at doses of 2 and 6 mg a. i. kg−1, repelled black flies for 1 and 2 days, respectively.

The toxicity of a number of topically applied pyrethroids has been tested against adult male desert locusts, Schistocerca gregaria: the most potent proved to be 5-benzyl-3-furylmethyl (+)-trans-chrysanthemate (bioresmethrin) with a weighted mean LD50 of 4.0 μg/g. The remaining compounds may be ranked in order of toxicity as follows: 5-benzyl-3-furylmethyl (±)-cis-trans-chrysanthemate (resmethrin) > 4-allyl-2,6-dimethyl-benzyl(+)-trans-chrysanthemate > 4-allylbenzyl (+)-trans-chrysanthemate > 2,4,6-trimethylbenzyl (+)-cis-trans-chrysanthemate > 2,3,4-trimethylbenzyl (+)-cis-trans-chrysanthemate > 2,4-dimethylbenzyl (±)-cis-trans-chrysanthemate; 2-methylbenzyl (±)-cis-trans-chrysanthemate. A small factor of synergism (4.2) was obtained with bioresmethrin following pre-treatment with sesamex, but with resmethrin the synergistic ratio (1.6) was of little practical significance.

A pyrethrins-resistant strain of houseflies, 213ab, previously selected with a 1:10 (by wt.) mixture of natural pyrethrins and piperonyl butoxide, was further selected either with natural pyrethrins alone (strain NPR) or with resmethrin (strain 104). After 50 generations the two populations differed in their resistance to the natural and synthetic esters. Both were resistant to all pyrethroids. Part of strain NPR was immune and very much more resistant than strain 104 to the natural pyrethrins and allethrin, but it was only 2–3 times more resistant than strain 104 against the new synthetic esters resmethrin (5-benzyl-3-furylmethyl (±)-cis-trans-chrysanthemate), bio-resmethrin (5-benzyl-3-furylmethyl (+)-trans-chrysanthemate), pyresmethrin (5-benzyl-3-furylmethyl pyrethrate) and 5B2Me3FC (5-benzyl-2-methyl-3-furylmethyl (±)-cis-trans-chrysanthemate). Pretreatment of both strains with sesamex diminished but did not eliminate resistance. Synergism was greater in strain NPR, especially with natural pyrethrins and allethrin. Both strains had great resistance to DDT indicating that resistance to DDT and pyrethroids is linked. Differences in resistance to different compounds suggest that at least three factors can confer resistance, one of which, pen, delays penetration and two others involve detoxication, one py a on the acid side of the ester linkage and the other, py b, on the alcohol side. Natural pyrethrins and resmethrin select for different groupings of these factors. Treatment with resmethrin does not select for py b presumably because this mechanism cannot attack the resmethrin molecule. Similarly when piperonyl butoxide is added to the natural pyrethrins py b is inhibited and so removed from selection pressure. Under these conditions, the strain produced contains the same factors as one selected by resmethrin and so shows the same small resistance to natural pyrethrins alone.

Pyrethrins and the pyrethroids, bioallethrin ((+) trans-chrysanthemic acid ester of (±) allethrolone), resmethrin* (5-benzyl-3-furylmethyl (±)-cis,trans-chrysanthemate) and bioresmethrin† (5-benzyl-3-furylmethyl (+)-trans-chrysanthemate) were compared for insecticidal activity against free-flying Aedes aegypti L. in a Peet-Grady chamber using kerosene solutions and an aerosol particle size. The relative potency for kill of female mosquitoes was 1; 2.3; 6.8; 8.0 respectively. In further experiments, resmethrin and bioresmethrin were compared as aerosol formulations in a Comet 4C aircraft using caged and fed female A. aegypti. Both compounds at 0.1% (by wt.) in pressure packs and 35 g of formulation per 100 m3 provided 99% kill. It is suggested that pressure packs containing 0.5% (by wt.) of either compound should be adequate for disinsection of passenger aircraft. However, bioresmethrin would appear to be the compound of choice in view of its greater biological efficiency under laboratory conditions, and consequent probable greater margin of kill in practice.

A simple laboratory technique for determining the activity of two pyrethroids (5-benzyl-3-furyImethyl DL-cis, trans-chrysanthemate and 5-benzyl-3-furylmethyl D-trans-chrysanthemate) against blackfly larvae (Simulium ornatum) is described. The results showed that the pyrethroids alone or with piperonyl butpxide were of similar activity to Abate (O,O,O',O'-tetramethyl O,O'-thiodi-p-phenylene phosphor-othioate) and Dursban (O,O-diethyl O-3,5,6-trichloro-2-pyridylphosphorothioate), insecticides of known activity against blackfly larvae. However, the pyrethroids cause rapid larval detachment in running water, and may be of value in controlling these insects when used in conjunction with another insecticide.