Chapter 26
New Selective Systemic Aphicides Richard M. Jacobson and M. Thriugnanam
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Rohm and Haas Company, 727 Norristown Road, Spring House, PA 19477
RH7988 is a fast-acting non-fumigant selective aphicide with contact and systemic action against a wide spectrum of aphid species. RH7988 is in full development by Rohm and Haas. It kills all species of aphids that have been tested. It controls organophosphate- and carbamate-resistant aphids with no deleterious effect toward beneficial insects. RH7988 shows excellent crop tolerance. It can be applied by soil, foliar, and seed treatments. The experimental shows excellent translaminar, xylem, and phloem translocation in plants. As it is downwardly systemic, application of RH7988 to the leaves of a plant protects the roots against root aphids. As it is upwardly systemic, application to the soil allows uptake into the roots and then dispersal throughout the plant. This combination of both upward and downward systemicity allows protection of the entire plant, including new growth, with either application method. With these unique properties, RH7988 should be an ideal fit with Integrated Pest Management programs. RH7988 is a potent aphicide. While its field use rates vary with crop, it is effective at field rates as low as 30 g per hectare. Mostfielduse rates are 60 g per hectare.
Aphids or plant lice are small (1-4 mm long) soft-bodied sucking insects that belong to the order Homoptera, Suborder Sternorrhyncha, Superfamily Aphidoidea, and the Family Aphididae. World-wide, they number about 4000 species under 224 genera and attack 97% of all flowering plants. Out of the reported 1351 aphid species, 80 are pests of food crops and ornamental plants (1, 2). They live on a single, alternate, or multiple hosts depending on the species. Aphids live on buds, flowers, fruits, leaves, stems, or even roots depending on the species and feed on phloem. Their feeding may cause stunting, discoloration, and deformation. They excrete honeydew that attracts sooty mold leading to impairment of photosynthesis and poor produce quality. Aphids transmit over one-half of all known plant viruses (3) (see Table-I).
0097-6156/91/0443-0322$06.00/0 © 1991 American Chemical Society
In Synthesis and Chemistry of Agrochemicals II; Baker, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
26. JACOBSON AND TIIRIUGNANAM
Selective Systemic Aphicides
323
TABLE-I IMPORTANCE OF APHIDS AS TRANSMITORS OF PLANT VIRAL DISEASES
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Vector Type
No. Vectors
No. of Viruses Transmitted
Aphids Leaf & Plant Hoppers Mealy Bugs White Flies Tingid Bug Psyllids
200 60 15 3 2 1
160 35 2 25 2 1
Total Hemiptera
281
225
Beetles Thrips Other Insects Mites Nematodes Snails Fungi
30 6 17 10 20 6 6
20 3 15 13 14 1 9
ALL TOTAL
376
300
Reprinted with permission from ref. 3. Copyright 1976 John Wiley & Sons.
Myzus persicae, the most important aphid vector is known to transmit over 100 plant viruses such as beet mild yellowing, beet yellow net, beet mild yellows, pea enation mosaic, potato leaf roll, radish yellows, tobacco vein distorting, and tobacco yellow vein banding (4). As a result of their ingestion, excretion, and disease transmission, aphids could cause serious crop loss. The best-known recent aphid damage is that of Russian Wheat aphid, Diuraphis noxia which was first discovered in Texas in 1986 but has now spread to 16 million of the total of 59 million wheat and small grain acres in the Great Plains states causing $37,000,000 annual crop loss (5). Due to the widespread use of synthetic pyrethroids killing beneficial insect predators, aphids are becoming increasingly important on cotton. 9
The need to control aphids with crop protection chemicals is evident from the difference in yield loss between treated and untreated crops (6). (see Table-II). A wide variety of insecticides are currently available for aphid control but an ideal insecticide should have new chemistry, biological selectivity, good mammalian toxicology profile, aphicidal spectrum and efficacy at low use rate, and safety toward beneficial insects. RH7988 has more of these desirable properties than the currently available aphicidal insecticides.
In Synthesis and Chemistry of Agrochemicals II; Baker, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
324
SYNTHESIS AND CHEMISTRY OF AGROCHEMICALS Π TABLE-Π CROP LOSS DUE TO APHIDS Average %Crop loss With Best Control With No Control
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Crop Alfalfa Barley Cole Crops Cotton Peas Pepper Potato Sorghum Tomato
1.3 1.0 1.5 0.3 40 0 0.5 2.8 0
48.8 7.0 56.0 22.3 48.7 24.0 15.0 19.4 6.5
CHEMISTRY OF RH7988 RH7988 is l-dimethylcarbamoyl-3-t-butyl-5-carboethoxymethylthio-l,2,4triazole, [I]. Neither a Commonname nor a Tradename has yet been decided.
RH7988
RH7988 can be synthesized on a laboratory scale by the following route. Treatment of trimethylacetyl chloride with thiosemicarbazide and sodium hydroxide cleanly gives the monoacylated thiosemicarbazide [II]. Further treatment with aqueous sodium hydroxide gives 3-t-butyl-5-mercapto-l,2,4triazole [III]
In Synthesis and Chemistry of Agrochemicals II; Baker, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
26. JACOBSON AND THRIUGNANAM
Selective Systemic Aphicides
Downloaded by UNIV OF GUELPH LIBRARY on June 22, 2012 | http://pubs.acs.org Publication Date: December 7, 1991 | doi: 10.1021/bk-1991-0443.ch026
Alkylation of the mercaptotriazole with ethyl chloroacetate puts in the critical side chain, yielding [IV]. Finally, treatment with dimethyl carbamoyl chloride, or phosgene followed by dimethylamine gives RH7988 [I].
This final acylation can, in theory, take place on any of the three nitrogens of the triazole. There is literature precedent for mixtures and even preference for another nitrogen in similar systems. We found essentially one isomer. While chemical intuition says that we expect the isomer shown, we could not be sure solely from the NMR. A single crystal X-ray crystal structure demonstrated unequivocally the structure is as depicted.
ACUTE MAMMALIAN TOXICOLOGY RH7988 is an acetylcholine esterase inhibitor. Considering the mode of action, it has a desirable acute toxicology profile. The technical material shows no dermal toxicity at the highest rates tested. Its acute oral toxicity in the mouse is 61 mg/kg making it moderately toxic. It is moderately irritating to rabbit skin and slightly initiating to rabbit eyes. It is non-mutagenic in the Ames test. More advanced toxicology shows a similar favorable outlook.
In Synthesis and Chemistry of Agrochemicals II; Baker, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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SYNTHESIS AND CHEMISTRY OF AGROCHEMICALS II
RESIDUE / METABOLISM ENVIRONMENTAL FATE Field scale residue and metabolism studies are underway for RH7988. We can summarize the laboratory studies as follows. The carbamoylated ester [I] is rapidly hydrolysed to the carbamolyated acid | \ ^ , \ [V]. The soil halflife is somewhat less than a day, \ / ο + Λ A^. \ Y i . = (/ O t1 / 2=
