|Controlling Insect Pests of Stored Products Using Insect Growth Regulators and Insecticides of Microbial Origin (NRI, 1994)|
|Section 3: Insect growth regulators: specific details|
Juvenile Hormone Analog I (JHA I)
(Zoecon Corporation/Sandoz A G)
Toxicology Acute oral LD50 for rats, >34 600 mg/kg ADI for man 0.1 mg/kg
(Standard formulations of methoprene usually contain a 1:1 mixture of R:S stereoisomers, the S-isomer being the biologically active form.)
Methoprene is a synthetic analogue of naturally occurring insect JH. It has recently been registered for use against insect pests of stored products. In 1989, draft maximum residue limits in cereal grains were advanced to Step 8 in the process of the Codex Alimentarius Commission (Codex) (Bengston et al., 1991 a). A maximum residue limit (MRL) of 5 ppm on cereal grain was recommended to Codex by the JMPR for 1988 (Samson et al., l 990).
Laboratory efficacy experiments
Loschiavo (1976) examined the effects of methoprene and hydroprene on the survival, development and reproduction of the following six species of stored product insects: Tribolium castaneum, T. confusum, Oryzaephilus surinamensis, O. mercator, Sitophilus granarius and S. oryzae. Methoprene and hydroprene were applied to appropriate insect diets at 1-20 ppm. The cultures were maintained at 30 i 1 °C and 63 + 3% relative humidity (r.h.); the numbers of F1 adults emerging were recorded.
Methoprene at 20 ppm prevented the development of larvae to pupae in T. castaneum and significantly reduced pupal production in T. confusum. At 5 ppm or higher, it inhibited oviposition in both species. Methoprene also increased the time taken for larval development and led to the production of super-larvae. Fully developed larvae of either species which had failed to pupate on food treated at 5-20 ppm had more sclerotized integuments and were larger, than those on untreated food. Larvae of O. mercator and O. surinamensis treated with 1-20 ppm died as pupal-adult intermediates. Production of F1 progeny in S. granarius and S. oryzae decreased with increasing concentration. However, at 20 ppm the reduction observed was not sufficient for practical control.
Shaaya et al. (1986) found that methoprene interfered with the development of eggs and larvae of Ephestia cautella in an age-dependent manner, and showed that only certain stages were susceptible. Young embryos prior to the stage of blastokinesis, larvae shortly before pupation, and fresh pupae, were most sensitive to methoprene applied to filter papers at 1 g/m2. Metamorphosis was inhibited and super-larvae were produced. Measurement of the ecdysone titre indicated that methoprene had inhibited the shift from rRNA synthesis to giant HnRNA which normally occurs in the epidermal cells during the transition from larvae to pupae.
Chakravorty et al. (1989) carried out tests on Corcyra cephalonica using methoprene and hydroprene at 1 -100 mg/individual. Methoprene-treated larvae became giant-sized, and some pupal-imaginal intermediates were produced which generally died. Many treated larvae were unable to spin cocoons prior to pupation and died.
The effects of topical application of methoprene to last instar
larvae and newly formed pupae of Sitotroga cerealella were examined by Stockel
and Edwards (1981). They found that application to maize grains containing
pupae, at a rate of 10 mg/pupa, resulted in 94% adult emergence. When the
same dose was applied to extracted pupae, adult emergence was 45% compared with
80-90% in the control. Adults which emerged following treatment with methoprene
appeared to be unaffected. Their reproductive capacity was also
El-Sayed (1984) applied methoprene to food media infested with Tribolium confusum and Trogoderma granarium at 0.5-10 ppm. Pupation was substantially reduced on media treated with 10 ppm. At 0.5 ppm, oviposition was inhibited, and pupal-adult intermediates and malformed adults were produced. Overall, T. granarium was more susceptible to methoprene than T. confusum.
Methoprene has been shown to be extremely stable, particularly when applied to commodities stored in dark conditions. The Zoecon Company, which manufactures the compound, treated 50 g samples of tobacco with 14C-methoprene and stored them in jars in the dark at 24°C and 64% r.h., for 4 years. Residue analysis indicated that little decomposition had occurred. Of the recoverable 14C content, 85% was 14C-methoprene, indicating that methoprene had remained largely stable for 4 years (Staiger et al., 1983).
Studies by Mian and Mulla (1982a) showed that the residual activity of methoprene at application rates of 5-10 ppm was sufficient to control Rhyzopertha dominica on stored wheat, barley and maize for a period of 12 months. They found that methoprene was less effective against Sitophilus oryzae, although after 12 months of treatment at 10 ppm, numbers had been reduced by 81 %. An application rate of 50 ppm was suggested for Sitophilus spp., but this was considered uneconomic. Subsequent studies showed that only 35-40% of the initial dose (1-10 ppm) remained after 12 months, although this was sufficient to control R. dominica (Mien and Mulla, 1983).
In Malaysia, Tan and Tan (1978) applied methoprene at 160 ng/cm2 to corrugated paper pupation sites. The paper sites were maintained at 30 + 1 °C in semi-darkness for various time intervals. They were then placed in plastic cups, containing 10 g of culture medium and 15-30 migrating larvae of Ephestia kuchniella, and maintained in semi-darkness at 25°C and 65% r.h. for 28 days. Methoprene caused a 72% reduction in adult emergence. Residue analyses indicated that over a 50-day period, the loss of activity of methoprene was 4.6% per 10-day interval. These results suggest that, under Malaysian conditions, the half-life of methoprene is sufficient to extend over five generations of E. kuehniella and offers suitable control provided ambient temperatures do not exceed 25-30°C.
In Australia, Samson et al. (1990) applied methoprene to hard wheat, yellow dent maize and long grain paddy rice with initial moisture contents of 13.2-17.2%. The grain was then infested with R. dominica and F1 production was monitored. The activity of methoprene on the three commodities was found to be unaffected by their initial moisture contents during the relatively short period of the experiment (one insect generation). It was suggested that this insensitivity to grain moisture may be advantageous in situations where grain is not fully dried before treatment.
Edwards et al. (1988) studied methoprene metabolism in pupae of a susceptible strain of Tribolium castaneum. Pupae aged between 0 and 18 hours were treated topically with 0.05 ml of a 1% solution of 14C-methoprene, and examined after 10 days. Methoprene was found to have a half-life of 18 h in 0-18 h pupae and was metabolized into three major products: 11 -hydroxy ester, 11 -methoxy acid, and 11-hydroxy acid derivatives. Pupae produced more 11-hydroxy ester derivative than 11-methoxy acid. The 11-hydroxy ester metabolite of methoprene retains considerable JH activity, whereas the 11 -hydroxy acid is inactive. It was suggested that the observed variation in sensitivity of insect species to methoprene could be due to differences in their ability to break down methoprene to inactive derivatives. For example, there is a marked difference in sensitivity to JHAs between the Tenebrionidae and Curculionidae. The metabolic pathway used by T. castaneum pupae by which the active metabolite, 11-hydroxy ester, is produced during the breakdown of methoprene, may contribute to its greater sensitivity to methoprene when compared with Sitophilus.
Miscellaneous biological effects
Other studies have indicated that JH is involved in controlling the production of aggregation pheromone in certain forest Coleoptera. The implications of this for the control of stored product pests were studied by Pierce et al. (1986). They found that when methoprene was tested against Cryptolestes ferrugineus, Oryzaephilus mercator, O. surinamensis and Tribolium castaneum at concentrations which would prevent F1 emergence, there were indications of enhanced aggregation pheromone production, and additional beetles were drawn to the treated product.
Methoprene at 5 ppm stimulated egg production in O. surinamensis although the eggs did not hatch, and caused almost complete mortality in eggs of Rhyzopertha dominica (Mien and Mulla, 1982a).
Topical applications of methoprene to virgin female Trogoderma granarium at rates of 0.02-0.5 mg, induced egg laying within 12-24 h. The number of eggs laid increased with methoprene concentration (Chellayan and Karnarvar, 1989). Normally, virgin females do not lay eggs.
Kucerova and Zuska (1991) examined the effect of methoprene on Liposcelis bostrychophilus when applied to a diet of ground soft wheat, ground oat flakes and dried brewers yeast in the ratio 5:5:1. Application of 100 ppm had no effect on pre-adult development, and 1000 ppm was required to prevent adult formation completely. However, Buchi (1991) tested the effect of methoprene on L. bostrychophilus at 0-190 ppm admixed to a food medium of bruised grain with 10% dried yeast. After 4 and 8 weeks, numbers of live adults on media treated at 47.5 ppm had been reduced by 73.5% and 31.5%, respectively, compared to the control. At 190 ppm, numbers of adults had been reduced by 96.7% and 84.6% after 4 and 8 weeks, respectively.
Hussein (1983) applied methoprene to broad beans at rates varying from 100 to 1600 ppm and examined the effect on oviposition and F1 emergence in Callosobruchus maculatus. Adult emergence from eggs laid 21 days after treatment was reduced by 48%, compared to the controls.
In Florida, United States (US), Vick et al. (1985) examined the effects of methoprene and the sex pheromone (Z,E)-9,12-tetradecadien-1-ol acetate on Ephestia cautella. Methoprene was applied at 10 ppm to peanuts stored in sections of a compartmentalized warehouse. For 9 months, it was effective in reducing feeding damage by E. cautella populations of 10 male and 10 female pupae introduced into each room twice weekly for 9 weeks. At the population densities tested, the effectiveness of the IGR treatment was not significantly enhanced by the presence of mating-disruptive doses of sex pheromone.
It was recommended that methoprene should only be used in situations of low infestation because the treated larvae feed intermittently over several months, producing a significant amount of damage. The need to evaluate the effectiveness of IGRs in terms of the reduction of damage to the commodity, rather than in terms of adult emergence, was emphasized.
IGR and insecticide combinations
Extensive studies are in progress in Australia on the effectiveness of IGR-insecticide combinations as long term grain protectants. A mixture of 1 ppm methoprene and 12 ppm fenitrothion was found to be effective, for 9 months, in controlling populations of Sitophilus oryzae, S. granarius, Rhyzopertha dominica, Tribolium castaneum, T. confusum, Oryzaephilus surinamensis and Ephestia cautella on wheat stored in unaerated silos (Bengston, 1987).
Bengston et al. (1991 b) carried out persistence trials in commercial stores at two sites in Australia. Bulk wheat was sprayed with emulsifiable concentrate (EC) formulations of S-methoprene (0.6 ppm) and fenitrothion (12 ppm) at a rate of 1 I/t during grain intake into concrete silos. The initial deposits of methoprene at both sites were approximately 80% of the calculated application rate; they persisted for the 9-month storage period, declining by only 20%. No natural infestations of insect pests were recorded during the trial. Laboratory bioassays using R. dominica and O. surinamensis were carried out on sub-samples of wheat over 8.5 months. A 100% reduction of adult F1 of both species was maintained for 8.5 months.
Arthur et al.(1990) compared the effectiveness of chlorpyrifos-methyl alone (6 ppm), and chlorpyrifos-methyl (6 ppm) mixed with methoprene at 1 ppm, when applied to maize in smalI bins. The treatments were tested against repeated artificial introductions of Cryptolestes pusillus, Plodia interpunctella, Sitophilus zeamais and T. castaneum. It was found that the addition of methoprene did not significantly improve the level of control achieved over a 12-month period. Both treatments effectively controlled the pest species, but it was suggested that the mixture would only be useful if the application rate of chiorpyrifos-methyl could be reduced, or if chlorpyrifos-methyl resistance developed.
Effect on seed germination
Kramer et al. (1985) found that an application rate of 10 ppm of methoprene reduced wheat germination by 13% 1 month after treatment.
Sandoz market a formulation of methoprene, called Diacon, for use in controlling stored product pests including Lasioderma serricorne, Ephestia cautella, Plodia interpunctella, Rhyzopertha dominica, and Oryzaephilus surinamensis.
In the US, Bulgaria and the UK, methoprene has been registered for use as a residual spray in food and feed handling establishments. In the US, it has also been registered for use on stored agricultural commodities for the control of major pest species excluding Sitophilus. Temporary MRLs have been established for methoprene on cereal grains, beans (dry), nuts, apples, apricots, raisins, peaches, pears, cocoa, coffee, spices, peas (dried), corn grits, hominy, macaroni and prunes (Bengston, 1987).
In Australia, methoprene has been cleared by federal authorities for use as a grain protectant and is registered in some states. It has been used effectively for protecting 100 000 t of wheat in commercial storage (Bengston, et al., 1991 b). However, concern has been expressed about the cost of methoprene application in pest management programmes, compared with that of traditional chemicals (Arthur et al., 1990).
The effectiveness of methoprene against most stored product pests, and its stability in stores, led to the development by the Zoecon Corporation of the KABAT Tobacco Protector, in which methoprene is applied at 10 mg/kg to control Ephestia elutella and Lasioderma serricorne. By 1987, the treatment of tobacco with methoprene represented the largest use of IGRs against stored product insect pests. This treatment has been registered, or cleared for use, in 29 countries, and has replaced phosphine fumigation in some places (Bengston, 1 987).
Dianex (methoprene 60% a.i.) has been developed and approved by
the EPA for crack and crevice treatments in warehouses. Laboratory experiments
on Trogoderma granarium indicated that the fecundity of virgin and gravid
females was significantly reduced after contact with paper treated with Dianex
at 100 mg/cm; males, however, were unaffected. High mortality was observed
in 0-1 -day old eggs which had been in contact with treated paper prior to
hatching. Corrugated cardboard provides an ideal oviposition and moulting site
for Trogoderma. Therefore, scattering strips of Dianex-treated cardboard through
a commodity was suggested as one means of control; treatment of wrapping papers,
packaging papers, pallet coverings and box-car linings was also suggested (Klein
and Burkholder, 1984).
Methoprene has been registered for the control of specific insects including Ephestia cautella, Plodia interpunctella, Lasioderma serricorne, Rhyzopertha dominica and Oryzeephilus surinamensis. Unfortunately, it does not control all pests and is unsuitable for control of Callosobruchus maculatus, Liposcelis bostrychophilus and Sitophilus spp. This limits its usefulness as a stored product protectant. To overcome this problem, trials are being carried out on the efficacy of methoprene applied in conjunction with conventional insecticides. An IGR-insecticide combination may also allow the use of a lower dose of the conventional insecticide.
(Zoecon Corporation/Sandoz A G)
Toxicology Acute oral LD50 for rats, >5000 mg/kg
Hydroprene is a chemical analogue of insect JH. It is effective against many species of Coleoptera, Homoptera and Lepidoptera, and is used commercially against cockroaches.
Laboratory efficacy experiments
McGregor and Kramer (1975) tested hydroprene against adult Plodia interpunctella, Rhyzopertha dominica, Tribolium confusum, Oryzacphilus surinamensis and Sitophilus oryzae. It was applied to wheat and maize at 2 and 10 ppm, and numbers of F1 were recorded. At 2 ppm on wheat, F1 adult emergence was reduced by 99%, 89% and 100% for R. dominica, O. surinamensis and T. confusum, respectively. When applied at 10 ppm, numbers of F1 S. oryzae were reduced by 35%. Treatment with 2 ppm prevented the normal development of P. interpunctella larvae; they grew into super-larvae and exhibited reduced webbing activity. Hydroprene was not toxic to the aduIt stages of any species tested.
Loschiavo (1976) tested hydroprene against Tribolium castaneum, T. confusum, Oryzaephilus mercator, O. surinamensis, Sitophilus granaries and S. oryzae. It was applied at 1-20 ppm to diet media, and the numbers of F1 and F2 progeny produced were counted. At 5 ppm, adult emergence of T. confusum was reduced by 85%; at this concentration, no T. castaneum, O. surinamensis or O. mercator emerged. At 20 ppm, a 99.6% and a 52% reduction in numbers of S. granarius and S. oryzae occurred, respectively. Loschiavo suggested that IGRs would only be suitable for controlling certain storage pests and for treating high value commodities stored in small quantities.
Topical application of 0.1-0.75 mg of hydroprene to unmated adult Callosobruchus maculatus did not affect female fecundity, although egg fertiIity was reduced. Those insects which developed to adulthood often showed morphological abnormalities, and the period needed for development increased by 8 days (Rup and Chopra, 1984). Application of hydroprene also reduced male fertility. Males treated with either 0.1 or 0.5 mg of hydroprene were mated with virgin females; compared with untreated pairs, the number of eggs laid was reduced by 91% and 98%, respectively.
Stockel and Edwards (1981) examined the effects of topical application of hydroprene on Sitotroga cerealella. With an application rate of 10 or 100 mg/ pupa, adult emergence was reduced by 92% and 100%, respectively. Topical application to maize containing pupae was less effective than direct application to the pupa. When applied at 10 mg/grain, adult emergence was only reduced by 52%.
Application of hydroprene to Corcyra cephalonica reduced the rate of egg laying and hatching (Chakravorty et al., 1986). Subsequent studies by Chakravorty et al. (1989) showed that topical applications of 1-100 mg to larvae and pupae inhibited or disturbed metamorphosis, growth, and differentiation of larval labial gland, gut and gonads. Pupal web spinning was largely inhibited, and production of normal eggs and viable sperm was affected.
Zoecon, the manufacturers of hydroprene, are intending to introduce a new formulation, called Gentrol, for crack, crevice and spot treatments against stored product pests in food processing areas in the US.
Like methoprene, hydroprene appears to have some practical potential, but storage pests are not all equally sensitive to the compound. Lepidoptera, especially S. cerealella, appear in laboratory tests to be relatively insensitive, thus limiting its possible range of use.
(Zoecon Corporation/Sandoz A G)
Toxicology Acute oral LD50 for rats, 4900 mg/kg
Kinoprene is another chemical analogue of insect juvenile hormone. It has been used in glasshouses to protect ornamental or vegetable seed crops against Homoptera such as aphids, mealybugs, scale insects and whiteflies.
Laboratory efficacy experiments
Shaaya and Pisarev (1986) exposed newly emerged Ephestia cautella larvae to media treated with either 6.5 or 65 ppm of kinoprene for 10 days; they were then transferred to IGR-free food. Kinoprene at 65 ppm prolonged the larval stage by only 9 days. Application of 6.5 or 65 ppm of kinoprene reduced adult emergence by 44 + 6% and 92 + 3.8%, respectively. An application rate of 65 ppm may, however, be uneconomic.
Kinoprene has been used commercially in glasshouses, but it is currently regarded as a compound which has been superseded and is of little commercial interest (Worthing and Hance, 1991). Research on its use as a stored product protectant appears to have been very limited.
(Dr R. Maag Ltd and Roche)
Toxicology Acute oral LD50 for rats, 16 800 mg/kg
Fenoxycarb is a non-neurotoxic, phenoxy-ethyl carbamate which exhibits JH activity against larvae and adults of many insect species.
Laboratory efficacy experiments
Edwards and Short (1984) carried out trials, using 5 g samples of wheat treated with fenoxycarb to assess its effect against adult Sitophilus spp. Fenoxycarb at 5 ppm reduced adult emergence in S. granarius to less than 2%; however, 10 ppm was required to reduce adult emergence in S. oryzae and S. zeamais to 4%.
Trials were undertaken in Australia to compare the effectiveness of fenoxycarb, methoprene and diflubenzuron against Rhyzopertha dominica when applied to wheat, maize and paddy rice at 25°C and 70% r.h. Aqueous solutions of the IGRs were pipetted onto the walls of glass containers holding 2 kg samples of commodity. The samples were then mechanically tumbled. Groups of SO R. dominica adults were introduced into sub-samples of 83 g; these were kept at 25°C and 70% r.h. for 26 days, and then at 30°C, until F1 and F2 progeny had developed into adults. This trial indicated that fenoxycarb was the least effective of the three IGRs, and that the minimum effective dose required to protect the commodity for a 48-week period would be 10 ppm on maize and 5 ppm on paddy (Samson et al., 1990).
Buchi (1991) tested the efficacy of fenoxycarb against Liposcelis bostrychophilus. At 4 ppm, fenoxycarb almost completely inhibited the formation of adult L. bostrychophilus (94% reduction in numbers of adults) for up to 8 weeks although super-nymphs were produced In Switzerland. where the investigation was conducted, fenoxycarb was regarded as a promising control agent against Liposcelis spp. in granaries.
Edwards et al. (1991) found that following an initial application of 8.2 ppm, 67% of the applied dose was still present after 2 years. Despite its persistence, the residues of fenoxycarb found in milled flour derived from grain treated at 4.2 and 8.2 ppm, were 0 and 0.5 ppm, respectively; the level in baked bread did not exceed 0.4 ppm throughout the trial. The residues in bran derived from wheat treated with 4.2 and 8.2 ppm reached a maximum at 3 months of 19.6 and 40.7 ppm, respectively, declining to 6.2 and 20.3 ppm at 24 months.
Effects against insecticide-resistant strains
Thind and Edwards (1986) examined the effect of fenoxycarb against susceptible and insecticide-resistant strains of insect pests. The order of susceptibility was Tribolium castaneum > Cryptolestes ferrugineus > 0ryzaephilus surinamensis >Rhyzopertha dominica. GeneralIy, complete inhibition of adult emergence was achieved with 1 ppm, although R. dominica required 5 ppm. A strain of T. castaneum (Kano-C) resistant to malathion was found to be more susceptible to fenoxycarb than a comparatively susceptible strain. However, some degree of increased tolerance to fenoxycarb was observed in a non-specific, malathion-resistant strain of T. castaneum (CTC12). No evidence of cross-tolerance to fenoxycarb was observed in the insecticide-resistant strains of C. ferrugineus, 0. surinamensis and R. dominica tested.
Ishaaya et al. (1984) showed that fenoxycarb could synergize the toxicity of the pyrethroids trans- and cis-permethrin (50:10 ppm), trans- and cis-cypermethrin (10:1 ppm), deltamethrin, and fenvalerate (1:50 ppm). When applied with 100 ppm of synergist (either fenoxycarb or piperonyl butoxide), the resultant toxicity was improved by factors of 1.5-2, and 4, respectively.
Application to surfaces
White (1987) examined the effect of fenoxycarb when applied to panels of wood, steel and concrete measuring 0.35 m2, at rates of 0.25 and 0.5 g/m2. Fourth instar larvae of Tribolium castaneum and Cryptolestes ferrugineus were kept in contact with the treated surfaces for 24 h,1 day after treatment, and at monthly intervals for 32 weeks, to investigate residual action. Steel- and wood-treated surfaces retained good residual action against both species. Treatments applied to concrete only remained active for 1 day. The high alkalinity of concrete may cause chemical breakdown of fenoxycarb by hydrolysis. Tests on wheat grains treated with 8 ppm fenoxycarb showed that treatment of one seed in four would give adequate protection of stored wheat against T. castaneum and C. ferrugineus. It was therefore concluded that fenoxycarb could be used to treat empty galvanized steel granaries. A rate of 0.25 g/m2 was thought to be adequate for long-term control of T. castaneum and C. ferrugineus, and it was suggested that a surface treatment would control populations in grain residues up to 2 cm deep. However, alternative formulations, such as impregnated dusts, were recommended for treatment of concrete floors.
Edwards et al. (1991) treated 0.6 t lots of wheat, kept at 25°C, with either fenoxycarb at 4.2 and 8.2 ppm, or chlorpyrifos-methyl at 3.9 ppm. Immediately after treatment, adults of four insect species were added to give an initial infestation of 300 adults (0.5/kg) for each species; a further 300 adults of each species were added 6 months later. Over a 12-month period, in both the fenoxycarb and the chlorpyrifos-methyl treated wheat, insect populations were generally held at less than 1 insect/kg, but Sitophilus granarius in fenoxycarb-treated wheat increased to 10.1 adult insects/kg, and Oryzaephilus surinamensis in chlorpyrifos-methyl treated wheat increased to 49 adult insects/kg. After 2 years, fenoxycarb at 4.2 ppm had continued to maintain populations of Tribolium castaneum, Rhyzopertha dominica and O. surinamensis below 1 insect/kg while S. granarius had increased to 37 adults/kg; at 8.2 ppm, populations of all four species had remained below 2.3 adults/kg. By contrast, in chlorpyrifos-methyl treated grain, populations of all four species had increased to more than 100 adults/kg. Unlike chlorpyrifos-methyl, fenoxycarb does not kill adult insects at concentrations of 4.2-8.2 ppm. Therefore, adults in fenoxycarb treated grain will remain active for at least 6 months before they die of natural causes. It was suggested that if fenoxycarb were applied together with chlorpyrifos-methyl (which is particularly effective against S. granarius), very good control of the major insect pests could be achieved under UK conditions.
Trials were undertaken in the US, using 50 kg lots of wheat in small containers, to compare the insecticidal effectiveness of fenoxycarb at 10 ppm, malathion at 10 ppm, and Bacillus thuringiensis (Dipel dust). Groups of 200 adults of Tribolium confusum and Sitophilus oryzae were introduced into the treated wheat; these were then sampled monthly for two grain storage seasons in Kansas where temperatures fluctuated between -2 °C and 49 °C. The following year, 250 eggs of Plodia interpunctella were added to each bin and insect populations recorded after 3 months. Treatment of wheat with 10 ppm of either fenoxycarb or malathion controlled both T. confusum and S. oryzae for two seasons. However, only fenoxycarb suppressed the P. interpunctella population, and then, only for 4 months. In contrast, B. thuringiensis, applied at a rate of 125 ppm raked into the top 10 cm of the bins, prevented the development of P. interpunctella. However, this treatment alone did not control 7: confusum and S. oryzae. Analysis of wheat seed, treated with fenoxycarb at 10 ppm after 12 months of storage showed that the compound was very stable with no loss of activity. The highest residue levels in milled fractions were found in bran; an initial level of 35 ppm had declined to 25 ppm at 12 months (Kramer et al., 1985).
Field trials were also undertaken in Texas, US over 18 months, on 80 kg lots of paddy rice. Following treatment with 5 and 10 ppm of fenoxycarb, or 14 ppm of malathion, the rice lots were placed in fibreboard bins in a metal store. Once or twice a month, 1000-5000 adults of Sitotroga cerealella, Rhyzopertha dominica, S. oryzae and Tribolium castaneum were sprinkled onto paper on the floor and allowed to move throughout the store. A 3 kg sample was removed from each bin, using a suction device, immediately after treatment and after 1, 3, 6, 9,12 and 18 months. The number of insects in a 500 g sample, and the number of insects emerging from the sample over the following 6 weeks, were recorded. A 2 kg sample was also taken for residue analysis. Live insects were found in all treatments, on all sampling occasions. However, no appreciable reproduction of any species occurred in rice treated with fenoxycarb. Residue analysis showed that for paddy rice nominally treated with 10 ppm, between 7 and 8 ppm were recovered from the whole grain immediately after treatment. The highest residues of 20-25 ppm were found on the hulls; these were retained over 12 months. Fenoxycarb accumulated in the rice bran fraction; it had reached a maximum of 4.5 ppm at 3 months and had declined to 3.4 ppm at 12 months. Residues in brown rice never exceeded 1.5 ppm and declined to 0.4 ppm after 12 months of storage; residues in milled rice never exceeded 0.4 ppm (Cogburn, 1988).
Effect on seed germination
It was reported by Kramer et al. (1985) that fenoxycarb does not affect the germination of wheat seed.
In 1986, the EPA registered fenoxycarb for use as a bait in the control of fire ants. The agency described fenoxycarb as having moderate acute toxicity to humans and non-target terrestrial organisms, and high toxicity to aquatic invertebrates.
Fenoxycarb appears to be a suitable post-harvest protectant, particularly for long-term storage. However, recent reports from Ciba-Geigy suggest that the manufacturers of the compound have stopped its further development as a protectant for stored grain because of its persistence on cereals and its resultant residues.
Juvenile Hormone Analog I (JHA 1)
Laboratory efficacy experiments
JHA I, when applied to diet media at 10 ppm, prevented pupation and adult emergence of Tribolium confusum. It did not affect feeding or the moult cycle of young larval instars, but the larvae moulted several more times to become super-larvae instead (Mkhize, 1983).
Similar effects were observed in trials against Tribolium castaneum reared on media treated with 10 ppm of JHA I. However, Rhyzopertha dominica failed to reproduce, and F1 adult emergence of Sitophilus oryzae was reduced to 8% (Mkhize, 1988).
Topical application of JHA I, at 2 ppm, to the cuticle of 1-day old last instar larvae or pupae of R. dominica, led to the production of larval-pupal or pupal-adult intermediates, al I of which died. In this trial, application of JHA I to wheat at 10 ppm reduced the production of R. dominica F1 adults to 10% (Mkhize, 1992).
Topical application of JHA I at 0.1 ppm, to the cuticle of 1-6 h old eggs of T. confusum reduced egg hatch to 2% compared with 96% in acetone-treated controls; application of JHA I, at 100 ppm, to 4-5 day old eggs did not reduce egg hatch (Mkhize, 1993).
JHA I has been shown to be effective against both internal and external grain feeders. Further laboratory and large-scale trials would need to be undertaken before it could be recommended as a candidate grain protectant.
(Stauffer Chemicals Co. Ltd/ZENECA Agrochemicals)
R-20458 is a JH mimic which was shown in laboratory and field trials to be useful for suppressing populations of the stable fly, Stomoxys calcitrans (Ivie et al., 1976). The possession of JH-mimicking properties led to the development of R-20458 in a number of Asian countries; it has been commercialized in China to increase silkworm productivity.
Laboratory efficacy experiments
Laboratory studies have shown that Sitophilus oryzae adults are
very tolerant of R-20458; dosage rates as high as 600 ppm, applied to a diet
medium, failed to induce total suppression of F1 progeny. At 100 ppm, the
reduction in adult emergence was 88%; at 600 ppm, a 98% reduction occurred.
R-20458 does not appear to interfere with either mating behaviour or fecundity
in Sitophilus, although it may reduce longevity. It did, however, suppress F2
progeny from F1 weevils which had developed in a treated medium. All the
offspring exhibited the same morphological abnormalities. At a treatment rate of
100 ppm, the reduction in adult emergence was 99.9%; a few normal F2 adults were
found. Nevertheless, S. oryzae was regarded as being highly tolerant of R-20458,
and although this product is not known to have a high mammalian toxicity,
application as a grain protectant at a rate of 100 ppm was considered
undesirable (Mkhize, 1986a).
Further studies by Mkhize (1988) using Tribolium confusum and T. castaneum showed that larvae reared in media treated with 10 ppm of R-20458 exhibit lethal morphological abnormalities and ecdysial failure. The larvae subsequently develop into larval/pupal and pupal/adult intermediates.
Mkhize and Gupta (1985) treated wheat with solutions of acetone, or 5% polyoxyethylene trioleate (Tween-85) in acetone, solutions containing 6.25,12.5 or 25 ppm of R-20458. Formulating the IGR in Tween-85 increased the effectiveness of R-20458 against S. oryzae. At 25 ppm R-20458, adult emergence was reduced by 57.8% with acetone alone and by 88.7% with Tween-85 in acetone. Tween-85 belongs to the group 'Tweens' which are non-ionic compounds stable in the presence of hormones. Their molecules have both lipophilic and hydrophilic functional units that enable the more hydrophilic Tweens to form oil-in-water emulsions with oily IGRs. This is thought to facilitate dispersal of IGRs on wheat kernels and to increase their penetration into the kernels.
Research on the application of R-20458 against stored product insect pests appears to be limited to the work of Mhkize and colleagues. Stauffer Chemicals developed and, patented the compound, but due to its small market potential, it was withdrawn. Stauffer Chemicals/ZENECA Agrochemicals have no current plans to develop the compound further.
Laboratory efficacy experiments
(Hoffman La Roche Inc.)
The efficacy of MV-678 was compared with that of diflubenzuron, triflumuron and methoprene (Mien and Mulla, 1982a). At 5 ppm, MV-678 was the least effective against first and late instar larvae of Rhyzopertha dominica, Oryzaephilus surinamensis and Tribolium castaneum.
Mkhize (1988) treated wheat with 10 ppm of MV-678, R-20458, JHA i and JHA II, and found that MV-678 was the least potent; it reduced adult emergence by 35% in Sitophilus oryzae and 48% in R. dominica. By contrast, JHA I, the most effective compound, reduced adult emergence of S. oryzae by 92% and that of R. dominica by 100%.
Studies over a 12-month period on wheat, maize and barley showed that MV-678 has a short residual life on stored grain. At 1-10 ppm, MV-678 only suppressed R. dominica infestations for up to 2 months (Mien and Mulla,1982b).
MV-678 appears to be non-persistent and of low efficacy. It is therefore unlikely to be evaluated further as a protectant for stored products unless short persistence is an over-riding requirement in particular circumstances.
(Stauffer Chemical Co Ltd/ZENECA Agrochemicals)
R-31026, is an intermediate product in the synthesis of thiocarbamate herbicides and has been shown to possess properties similar to those of methoprene.
Laboratory efficacy experiments
Studies have shown that when R-31026 is applied to newly formed pupae of Tribolium confusum and T. castaneum, the formation and differentiation of tissues and organs is affected, and adult development is disrupted. When larvae of both species were reared on a diet treated with R-31026 at 20 mg/kg, 95% developed into pupa-adult intermediates.
Unlike many other potential IGRs, R-31026 does not prolong the larval feeding stage and is therefore considered to be suitable for further trials, particularly in view of its very low mammalian toxicity of >2300 mg/kg (Ishaaya, 1982).
Although this compound showed promise as an IGR, no further research appears to have been done on it . ZENECA Agrochemicals, the current holders of the patent, have no interest in developing the product further at the present time.