Biodegradable Pesticides in Public Health Entomology
219
Florida. Poor coverage at some stations where mosquito landing rates were taken
was attributed to dense vegetation and/or shifts in wind direction during application. Although this resulted in a low mean percent reduction (Table 8.6), the
authors stated that naled and the higher rate of propoxur (0.038 lb/acre) gave
complete control at many of the landing stations.
Bioresmethrin, a synthetic pyrethroid, proved effective against Ae. taeniorhynchus when applied by ground equipment as a UL V cold aerosol to dense
mangrove swamp on Grand Cayman (Brooke et al., 1974) (Table 8.6).
Against a sedentary mosquito such as Ae aegypti. successful control has
been obtained with applications made at wider intervals. Eliason et al. (1970)
found that oviposition by Ae. aegypti was almost totally interrupted by aerial
applications of malathion at 3 fl ozlacre twice per week. The premises index
was 20.1 % prior to treatment but only 1.3% at the end of 25 treatments. The
posttreatment index in the control area was 17.9%.
Based mainly on experience with malathion, UL V sprays appear generally
safe to nontarget aquatic organisms (Tagatz et al .• 1974; see exceptions cited
by these authors) and to nontarget terrestrial organisms (Hill et al .• 1971). The
latter, however, observed significant reductions of Homoptera and Hymenoptera
associated with spraying and substantial apiary losses occurred when hives were
not covered. Caron (1979) reported that daytime applications of malathion and
naled were more harmful to bees than was pyrethrins, while none of the insecticides had a discernible effect on bees when applied at night.
8.7.2. Synthetic Pyrethroids
The photostabilization of pyrethroids has opened new horizons for their use
as agricultural and public health insecticides. High cost and lack of persistence
limited their use in the past despite their high insecticidal activity, low mammalian
toxicity, and rapid biodegradability. Based on published acute LDso values (mg/
kg) for rats and four species of insects (topical application), Elliott (1977) calculated a general ratio of insect-to-mammal toxicity of 4500 for pyrethroids
compared with 91 for OC compounds, 33 for OP compounds, and 16 for carbamates.
A main feature of pyrethroids is that they are flexible molecules, and as
knowledge of structure-activity relationships has increased, it has been possible
to devise insecticides of greatly increased insect toxicity and stability without
sacrificing low mammalian toxicity (Elliott et al .• 1973; Miyamoto, 1976; Verschoyle and Bames, 1972) or environmental biodegradability (Barlow et al .•
1977; Kaufman et al .• 1977). For example, decamethrin, a compound stable
enough for field use, shows an LDso of 0.0003 .... g per female house fly compared
with 0.33 .... g for pyrethrin I, a looo-foid increase in potency (Elliott, 1977).
Decamethrin is 600 times as active as DDT to An. stephensi (Barlow et al .•
219
Florida. Poor coverage at some stations where mosquito landing rates were taken
was attributed to dense vegetation and/or shifts in wind direction during application. Although this resulted in a low mean percent reduction (Table 8.6), the
authors stated that naled and the higher rate of propoxur (0.038 lb/acre) gave
complete control at many of the landing stations.
Bioresmethrin, a synthetic pyrethroid, proved effective against Ae. taeniorhynchus when applied by ground equipment as a UL V cold aerosol to dense
mangrove swamp on Grand Cayman (Brooke et al., 1974) (Table 8.6).
Against a sedentary mosquito such as Ae aegypti. successful control has
been obtained with applications made at wider intervals. Eliason et al. (1970)
found that oviposition by Ae. aegypti was almost totally interrupted by aerial
applications of malathion at 3 fl ozlacre twice per week. The premises index
was 20.1 % prior to treatment but only 1.3% at the end of 25 treatments. The
posttreatment index in the control area was 17.9%.
Based mainly on experience with malathion, UL V sprays appear generally
safe to nontarget aquatic organisms (Tagatz et al .• 1974; see exceptions cited
by these authors) and to nontarget terrestrial organisms (Hill et al .• 1971). The
latter, however, observed significant reductions of Homoptera and Hymenoptera
associated with spraying and substantial apiary losses occurred when hives were
not covered. Caron (1979) reported that daytime applications of malathion and
naled were more harmful to bees than was pyrethrins, while none of the insecticides had a discernible effect on bees when applied at night.
8.7.2. Synthetic Pyrethroids
The photostabilization of pyrethroids has opened new horizons for their use
as agricultural and public health insecticides. High cost and lack of persistence
limited their use in the past despite their high insecticidal activity, low mammalian
toxicity, and rapid biodegradability. Based on published acute LDso values (mg/
kg) for rats and four species of insects (topical application), Elliott (1977) calculated a general ratio of insect-to-mammal toxicity of 4500 for pyrethroids
compared with 91 for OC compounds, 33 for OP compounds, and 16 for carbamates.
A main feature of pyrethroids is that they are flexible molecules, and as
knowledge of structure-activity relationships has increased, it has been possible
to devise insecticides of greatly increased insect toxicity and stability without
sacrificing low mammalian toxicity (Elliott et al .• 1973; Miyamoto, 1976; Verschoyle and Bames, 1972) or environmental biodegradability (Barlow et al .•
1977; Kaufman et al .• 1977). For example, decamethrin, a compound stable
enough for field use, shows an LDso of 0.0003 .... g per female house fly compared
with 0.33 .... g for pyrethrin I, a looo-foid increase in potency (Elliott, 1977).
Decamethrin is 600 times as active as DDT to An. stephensi (Barlow et al .•