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Movement and residual toxicity of carbofuran in acid mineral soil
Agriculture, Ecosystems and Environment, 9 (1983) 57--68
57
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
MOVEMENT AND RESIDUAL TOXICITY OF CARBOFURAN IN ACID MINERAL SOIL*
D.C. READ ~ and S.O. GAUL 2
Agriculture Canada, Research Station, Charlottetown, Prince Edward Island CIA 7M8 (Canada) 2Agriculture Canada, Research Station, Kentville, Nova Scotia B4N 1,]5 (Canada) (Accepted 6 October 1982)
ABSTRACT Read, D.C. and Gaul, S.O., 1983. Movement and residual toxicity o f carbofuran in acid mineral soil. Agric. Ecosystems Environ., 9 : 57---68. In greenhouse miniplots with soil moisture moving upward from the subsoil, toxicants of carbofuran moved upward and became detectable at the soil surface in ca. 3, 7, and 28 days after 10% granules of the insecticide were banded 1, 3, and 10 cm, respectively, below the surface. The test soil had a pH of 5.2 and contained 2.9% organic matter. Toxic residues reaching the soil surface from a depth of 5--10 cm increased over a period o f 8--12 months and gradually decreased thereafter. Fifteen to 20 ppm in the surface soil was highly toxic to flies contacting the soil surface. A m o u n t s in excess o f 40 ppm in the surface layer caused knockdown in 10--15 and estimated death in less than 30 s. The LDs0 for cabbage maggot eggs on the surface of treated soil was 8.2 ppm and for larvae 0.86 ppm. Chemical analysis records correlated well with those o f bioassay tests. In field microplots, the addition o f water during rainstorms (giving a total of 50 m m ) demonstrated that toxic residues of carbofuran could be leached downward to at least 45 cm. However, under the normal weather conditions of 1980 and under abnormally dry conditions most o f the detectable residues remained near or above the point o f application. INTRODUCTION
Carbofuran has been used extensively for more than a decade to control soil and foliage insect pests of crops. Although several recent studies have been concerned with the influence o f micro-organisms on the rate o f degradation (see Felsot et al., 1981), and Getzin (1973) has observed that the c o m p o u n d degraded much more rapidly in alkaline than in highly acidic soils, little attention has been given to m o v e m e n t and persistence o f toxic residues in soil. Many o f the studies on the persistence of carbofuran have involved labora. tory tests conducted with technical or reference grade material thoroughly *Contribution No. 501, Agriculture Canada, Research Station, Charlottetown; and No. 1765, Agriculture Canada, Research Station, Kentville.
0167-8809/83/$03.00
© 1983 Elsevier Science Publishers B.V.
58 mixed with the soil and it is difficult to relate results of such tests with persistence or movement of residues of commercial granular formulation as applied for control of soil insects. The significance of this problem was clearly demonstrated by Ahmad et al. (1979) who found that technical carbofuran showed 50% degradation in 7 days in the same soil in which a 10% granular formulation degraded only 25--35% in 35 days. Fuhremann and Lichtenstein (1980) studied the persistence and movement of carbofuran and other compounds in soil, but t h e y too used labelled and pure chemicals mixed thoroughly with their soils. The present study was designed to determine the m o v e m e n t and persistence of commercial grade 10% granular carbofuran applied at different depths in acid mineral soils n o t previously treated with pesticides. M AT ER I ALS AND METHODS
The soil used in the microplot and miniplot tests comprised a mixture of a sandy soil and a fine sandy loam to provide a texture which minimized the problem o f baking and cracking after surface watering. The soil was collected in bulk lots (ca. 1000 kg during September of each year) from fields n o t previously treated with pesticides, screened to remove stones and plant roots, and thoroughly mixed. The mixture, containing 2.9% organic matter and 11% clay and having a pH of 5.2, was stored prior to testing during the fall, winter, and spring in open-top plastic bags in an unheated storage building. A 10% granular formulation of carbofuran was used in all of the experiments. The concentration applied to the soil was estimated at more than twice t h a t normally used for field applications using a subsurface band or seed furrow t r e a t m e n t for vegetable insect control. A commercial application of 3 kg AI ha -1 in a 10-cm wide subsurface band may be estimated to be roughly 46 ppm in the upper 5 cm of soil. The a m o u n t applied in most of these experiments was established at 100 ppm in the 5-cm depth. Higher and lower concentrations were also used for specific tests. All of the miniplot and microplot experiments were replicated at least three times or a series of miniplots of one t r e a t m e n t was established to allow testing of three replicates at interval times of sampling. Miniplot tests were conducted using 6-cm square standard plastic drain pipe cut into 15-cm lengths. For greenhouse studies, series of the square plastic miniplots were filled with the test soil and set up on a base o f 3 cm o f soil overlying 5 cm of sphagnum peat moss on a greenhouse bench. The insecticide was banded at the required depth below the soil surface in the miniplot. Fifteen cm diameter miniplots and the 6-cm square type were also used for testing the m o v e m e n t of subsurface spot applications of carbofuran. The temperature in the greenhouse c o m p a r t m e n t was maintained at 20 + 2°C. Sunlight intensity was restricted using 15-cm wide strips of styrofoam placed 15 cm apart and running north to south across the top and south side o f the c o m p a r t m e n t .
59 A field miniplot comprised a 45-cm depth of soil in three o f the 15-cm long sections of plastic pipe taped together with waterproof tape to facilitate separation of different depth layers at the time of sampling. Replicates of these miniplots were imbedded vertically in the soil with ca. 1 cm of the plastic protruding above the surrounding soft. Carbofuran was applied 3 cm below the soil surface. Testing for toxic residues at different depths in the soil involved sliding the block of soil from the 15-cm long sections o f the miniplots and separating 1 or 5-cm depth layers as required for analyses. The soil in the greenhouse miniplots received moisture from the underlying moistened soil-over-moss base or b y sprinkling water over the surface. In field tests, one set o f three replicated miniplots was exposed to normal weather conditions while additional water was added to a second replicated set to give 'above normal' rainfall. For the field microplot tests, soil was encompassed within a 3
60 maggot. For the adulticide tests 300--500 flies were released into cages conraining a series of replicated miniplots in which insecticide had been banded at different depths below the soil surface. The number of flies killed due to contact with the soil surface gave an indication of the levels of toxicant reaching the surface within a given time from insecticide applications at different depths. The p h e n o m e n o n of the high p o t e n c y of certain insecticides in killing flies in contact with the surface of treated soil, from an insecticide either mixed into the surface soil or applied at various depths below the surface, is discussed in detail in previous papers on aldicarb and p r o p o x u r (Read, 1981a, b). Records of tests on subdivided layers of soil removed from the miniplots at different times after toxicants became detectable at the soil surface gave an estimate of the levels of toxic residues at different levels above and below the point of application. Ovicidal and larvicidal toxicity was measured by placing 100 eggs (ca. 36 h prior to hatching} in 5-cm diameter petri dishes containing 5 g of a sample of test soil. A small rectangular slice of rutabaga was placed in the centre o f each petri dish between t w o lots of 50 eggs as food for surviving larvae. Records on egg and larval mortality and tunnels in rutabaga slices were taken 24 h after egg hatch. Initial toxicity tests with several insecticides mixed with soil showed that organophosphorus and carbamate materials at 0.2--15 p p m caused ca. 5--99% mortality of larvae, while levels of 10--80 p p m caused ca. 5--99% egg mortality. In situations where a treatment caused 100% kill of eggs, tests were repeated with serial dilutions of the test soil mixed with untreated soil to enable calculation of the approximate levels of toxic residues in the treated sample. Confirmation of levels of residual toxicants of carbofuran in soil from selected tests was made by comparing the results of bioassay tests with those of chemical analysis. The portions of the samples from the greenhouse miniplots or field microplots to be tested for chemical analysis were air-dried and stored a t - 2 0 ° C prior to extraction and analysis. Chemical analysis involved the gas--liquid chromatographic (GLC) determination of carbofuran from soil extracts with an alkali flame ionization detector. Extraction of soils (50.0 g or the total sample) followed the m e t h o d of Robinson and Chapman {1980}. The chloroform extracts were evaporated to a small volume, made up to volume (15 ml} in ethyl acetate, and injected (2 ~1} directly. To increase detector stability, clean-up of the extract using a fast wool plugged Pasteur pipette column containing 5 cm Florisil t o p p e d with 1 cm anhydrous sodium sulfate was carried out. One ml of soil extract was added to the pre-rinsed column followed by 10 ml ethyl acetate. The eluant was reconcentrated to 1 ml before GLC injection. A Tracor Microtak 220 GLC was used equipped with a Model 702 Nitrogen-Phosphorus detector. The column was glass (2 mm id × 60 cm) packed with 1.5% OV-17 + 1.95% OV-210 on Chromosorb WHP, 80--100 mesh. Operating conditions were: oven temperature, 143 ° C; inlet temperature, 200 ° C; d e t e c t o r temperature, 285°C; carrier gas (helium} flow rate, 80 ml min -~ ;
61 hydrogen 2.2 ml min-1; compressed air, 60 psig at tank; electrometer attenuation, 1 X 8 or 1 × 16. O u t p u t was recorded on a Westronics/mv f.s.d, recorded run at a chart speed of 1.27 cm min -1 . Alternate injections of standard, sample, and appropriate standard were made and the ca~bofuran concentration determined on the basis of peak heights. The limit of detection with the operating conditions used was 0.04 p p m and recovery from spiked samples with 0.5--1.5 p p m ranged from 85 to 102%. Results are reported only for carbofuran. No 3-hydroxy carbofuran was detected in any of the samples and tests were n o t conducted for residues of the 3-keto metabolite. Miles and Harris (1981) demonstrated that the two metabolites comprised a relatively small percentage of the total carbofuran residues present in the soil. RESULTS
The data in Table I show the adulticidal toxicities o f carbofuran compared with those of six other insecticides and demonstrate that the adulticidal c o m p o u n d s were continually moving toward the surface of the soil in the greenhouse miniplots. During the period of peak toxicity for a c o m p o u n d , as at 11--20 days for the 1-cm depth o f aldicarb, k n o c k d o w n o f flies contacting the soil surface occurred in 10--15 s and detectable twitching of legs and wings had ceased in less than 30 s. Other adulticide bioassay tests with applications at depths of 5 and 10 cm demonstrated that insecticides applied at the lower depths did n o t reach the high concentrations at the soil surface as attained from 1 to 3 cm depths. However, toxic residues persisted longer. Base-line toxicity data for laboratory tests on carbofuran as an ovicide and larvicide are presented in Table II. These records show that 2--40 p p m caused ca. 1--100% mortality of eggs and concentrations of 0.2--3 ppm resulted in 0--100% mortality o f test larvae. Records of toxicity tests c o n d u c t e d at different times after soil application of carbofuran in greenhouse tests were compared with the data in Table II to obtain estimates of toxic residues remaining in the soil. The results presented in Table III demonstrate the correlation between bioassay and chemical analysis. These data show the relative amounts of toxic residues persisting at different levels in the soil 75 days after carbofuran was banded at a depth of 10 cm. The highest levels had become concentrated near the soil surface and residues near the point of application were greatly reduced. After 200 days, all detectable residues were concentrated near the surface and no toxicant could be detected at or near the original point of application. Records of the greenhouse bioassay tests on long-term persistence (Table IV) indicate that toxic residues were continually moving toward, and concentrating at, the surface. Results of a series of experiments (data n o t included), with tests c o n d u c t e d after 25, 50, 100, and 150 days, supported the data in Tables III and IV on the upward m o v e m e n t of toxic residues. Results of chemical analysis averaged for the nine layers of soil for which
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63 TABLE II Initial percentage mortalities o f cabbage maggot eggs and larvae caused by different concentrations of carbofuran in acid mineral soil and tested in 5-g samples of treated soil in 5-cm diameter petri dishes a Insecticide in soil (ppm)
50 40 30 20 15 10 5 2 1 0.8 0.4 0.2 Untreated
Mortality b (%)
Tunnels in rutabaga
Eggs
Larvae
(slices by surviving larvae)
100 100 98 82 77 61 24 14 8 5 5 5 5 5
100 100 100 100 100 100 100 100 87 62 27 14 0 0
0 0 0 0 0 0 0 0 13 34 Moderate Extensive Extensive Extensive
aAverages for four tests conducted at different times (with different lots of eggs) with two replications of each test. bEggs incubated on treated soil for at least 36 h prior to hatching. Larvae from eggs placed on treated soil ca. 2--6 h prior to hatching.
TABLE
III
Carbofuran residues at different levels in the soil 75 days after the insecticide was banded at a depth of 10 cm in miniplots at 20 ppm in the total volume of soil Depth layer of soil (cm)
0--1 1--2 2--4 4--6.5 6.5--9 9--11 b 11--12 12--13 13--15
Carbofuran residues (ppm) a Chemical analysis
Bioassay estimates
27.68 7.95 5.38 2.13 2.98 6.34 ND 0.79 0.23
32 9 6.3 1.5 4.5 7.5 ND 1.2 ND
aND = non-detectable residues by chemical analysis -- less than 0.04 ppm; by bioassay -no larval mortality. bDepth layer containing original application, estimated to have contained 150 ppm carbofuran at the time o f application.
64 TABLE IV Percentage mortalities of cabbage maggot eggs and larvae and estimated residues in ppm of carbofuran 200 days after insecticide was banded at a depth of 10 cm in 6-cm square miniplots to give 30 ppm in total volume of soil
Soil depth tested (cm)
0--1 1--2 2--4 4--6 6--8 8--9 9--11 b 11--13 13--15 Untreated
Mortality (%) Eggs
Larvae
99 36 17 11 8 3 5 4 5 4
100 100 92 43 7 0 0 0 0 0
Estimated level of residues (ppm) a
30 6 2.5 1 0.5 ND ND ND ND ND
aND = none detectable. Data determined from a Log--Probit curve prepared from data used for Table II. b Location of original band of insecticide (at 10-cm depth). TABLE V Upward and horizontal movement in soil from a coin-sized (1.25-cm diameter) con-centrated application of 10% carbofuran granules placed at a depth of 7.5 cm below the surface and in the centre of 15-cm diameter miniplots and analysed after 55 days a
Depth of soil sample (cm)
0--2 2--4 4--6 6--8 8--10
Concentration of carbofuran (ppm) b 'Centre'
'Adjoining'
'Outer'
12.88 5.72 9.96 2513.90 3.52
6.06 1.41 1.23 1.13 ND
1.28 0.71 ND ND ND
aChemical analysis data. Bioassay estimates correlated closely with these data. bResidues at different depths and from 2.5-cm diameter cores of soil taken in the 'centre' (directly over the original concentrated application), two 'adjoining' cores, and four ' o u t e r ' cores located around the centre core and 0.5 and 3.5 cm, respectively, away from the outer edge of the centre core. d a t a o n t h e d i f f e r e n t l a y e r s is g i v e n i n T a b l e I I I s h o w e d a r e s i d u e o f 5 . 8 4 p p m . T h i s r e p r e s e n t s t h e r e s i d u e i n t h e t o t a l v o l u m e o f soil a n d i n d i c a t e s a d e g r a d a t i o n o f ca. 7 0 % d u r i n g t h e 7 5 - d a y t e s t . In greenhouse experiments using 15-cm diameter and 15-cm deep miniplots in a d d i t i o n to the 6-cm square m i n i p l o t s placed over t h e soil-moss base,
65
but with a concentrated (1.25-cm diameter) spot application of 10% carbofuran granules placed in the centre and 7.5 cm below the soil surface, the toxicant moved directly upward in the centre of the miniplots and horizontal movement appeared to be limited. The results of one replicated experiment, which was sampled and analysed after 55 days, are presented in Table V. Bioassay tests conducted with similar central subsurface spot applications and tested at intervals over a period of 2.5 years showed similar trends of TABLE VI Residues (ppm) of carbofuran in different depth layers of soil 90 and 130 days after soil treatment at a depth of 2.5 cm in field microplots and miniplots receiving 'below normal', 'normal', and 'above normal' amounts of rainfall during the growing season as measured by chemical analysis and bioassay a Depth of soil layers (cm)
Rainfall regimen for first 90 days in microplots Below normal (155 mm)
Normal (265 ram)
Above normal (310 mm)
Residues in triplet layer microplots within aluminum frames (12 × 25 cm)
0--5 5--10
10--15
Chemical
Bioassay
Chemical
Bioassay
Chemical
Bioassay
68.25 0.57 0.56
76 Trace Trace
73.0 1.91 0.98
61.4 3.4 2.6
----
53.5 6.5 1.5
Rainfall regimen for 130 days in microplots b
0--5 5--10 10--15 15--20 20--25
Below normal (210 ram)
Normal (350 ram)
Above normal (435 mm)
48.13 0.87 0.73 0.17 ND
48.49 3.38 0.85 ---
47.14 0.81 0.26 0.14 0.58
43.5 1.5 0.4 0.6 0.8
20.99 3.77 1.22 0.82 0.78 0.93 0.88 0.78 0.29
18.5 4.2 1.0 0.8 0.8 1.1 0.6 0.4 ND
59 1.4 ND ND ND
55.0 4.5 1.5 ND ND
Residues in 45-cm deep miniplot column after 130 days 0--5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45
35.20 4.26 0.13 0.45 0.15 0.37 Trace ND ND
38.45 4.8 0.8 0.5 0.5 ND 'N D ND ND
aOriginal application in the 0--5 cm depth was 115 ppm in the microplots and 100 ppm in the miniplots. bSamples at 15--20 and 20--25 cm depths were core samples from the subsoil under the microplots.
66
upward movement, limited horizontal spread, and also of extreme persistence of the carbofuran in the concentrated central core containing the original application. Two years after applying such a spot application (calculated at 3000 p p m in 10 g of soil immediately surrounding the carbofuran granules and situated between 7 and 8 cm below the soil surface), toxic residues of ca. 600 p p m still remained in the central 10-g core. Residues in the 0--6-cm depths directly above the granules remained at high levels for 10--14 months, especially near the surface, and then gradually decreased. Data from field microplot tests c o n d u c t e d during the growing season (midJune to late-October) indicated that 'below normal' rainfall conditions resulted in a slightly faster rate of degradation of carbofuran than where the surface soil was fully exposed to normal rainfall (Table VI). Nevertheless, most o f the residues in the 'normal' and 'below normal' rainfall microplots remained in the 0--5-cm layer in which the carbofuran had been applied. However, additional water added to the 'above normal' microplots increased d o w n w a r d movement of toxic residues. This downward m o v e m e n t was further demonstrated in the 45-cm deep field miniplots where residues were found at all depth levels. It is significant to note that an apparent state of equilibrium occurred between 15 and 40 cm below the soil surface, with all five levels showing residues of 0.85 + 0.07. DISCUSSION
The results of these greenhouse and field experiments demonstrate that carbofuran is quite mobile in soil and that subsurface band or furrow applications, in general, tend to move upward and become concentrated near the soil surface. Since this trend is correlated with movement o f soil moisture, it is assumed that subsurface insecticide applications in ridged rows in a field would always tend to move upward because excessive rainwater from a heavy rainfall would run o f f the sides of the rows. This could explain w h y a single pre-planting application of relatively non-persistent insecticides, such as carbofuran or propoxur, has given all-season protection of rutabagas against r o o t maggot infestations. Such persistence was particularly apparent during a relatively dry growing season when p r o p o x u r gave slightly better r o o t maggot control than carbofuran. By comparison, p r o p o x u r gave no apparent protection during abnormally wet seasons (Read, 1976, 1981b). With ground level plantings, and particularly where the packing wheel behind the seeder could make a shallow trench over the insecticide and seed, the records in Table VI suggest that heavy rainfall could leach carbofuran residues d o w n below the point of attack by insects. Therefore, pre-planting seed-furrow or subsurface band treatments would be expected to give poor residual control during abnormally wet growing seasons. Several articles have recently been published on enhanced degradation of carbofuran in soil containing adaptive carbofuran
67 that the ramifications of enhanced microbial degradation in 'microbe' soil have intentionally been excluded from this report, especially the influence under high moisture or essentially anaerobic conditions (see Gorder et al. (1982) for references). Our recent studies have shown that the carbamate insecticide, carbofuran, added to 'carbofuran microbe' soil and the organophosphorus insecticide, fensulfothion, added to 'fensulfothion microbe' soil (resulting from repeated field applications of the insecticide to the same soil) degrade much more rapidly than in previously untreated or 'normal' soil and under normal field moisture conditions. The use of 'microbe' soils during this study with carbofuran caused variations in persistence and increased the complexity of the basic study on movement. Nevertheless, even in soils k n o w n to contain these micro-organisms, the pattern of m o v e m e n t of toxicant was the same as described in this paper. The most significant difference was that the persistence of toxicants varied greatly at different levels in the soil profile and was retarded in air dry soil. In tests with 'microbe' soil, set up in both the 6-cm square and the 15-cm diameter miniplots, the initial m o v e m e n t from a subsurface band application followed precisely the same trend toward the surface as for the 'normal' soil (data given in Table III). When banded at a depth of 7.5 cm, carbofuran initially moved upward, as in the 'normal' soil, and became concentrated near the surface. However, residues disappeared more quickly from the region near the point of application and after 50--60 days trace amounts of toxicant could be detected only in the t o p few millimeters of soil. The same initial upward m o v e m e n t occurred when spot concentrations of carbofuran granules were placed 7.5 cm deep and in the centre of miniplots o f 'microbe' soil. High concentrations of the insecticide were detected at the surface in 15--20 days. However, after ca. 30 days all traces of residues had disappeared from all of the upper layers of soil and after 50--60 days toxicant could only be detected at and just above the point where the spot concentration had been applied. Breakdown was more rapid in the 15-cm diameter miniplots than in the 6-cm square miniplots and it was assumed that the adaptive degrading microorganisms were moving inward from the sides of the miniplots and breaking d o w n the carbofuran as it was moving upward from the centre spot application. The micro-organisms were apparently unable to rapidly degrade the spot application (ca. 50 000 p p m in the 1.25-cm diameter core). Subsequent tests have confirmed the above observations and results will be reported in detail later. It is important to note at this time that even the enhanced microbial degradation was essentially eliminated in highly acid soil (below a pH of ca. 4.5). Hence, both chemical breakdown, as demonstrated by Getzin (1973), and microbial degradation, normal or enhanced, are greatly retarded in.highly acid soils. For soils with pH values of 4.3 and 6.0, Getzin (1973) found carbofuran to have a relatively long half-life (more than 50 weeks in highly acid soil). In our 'microbe' soil with pH values in the range of 6.4 d o w n to 5.1, adaptive micro-organisms could degrade 1000 ppm of carbofuran in less
68
than 30 days, but were essentially inactive at pH levels of 4.5 or lower. Since most highly acidic mineral soils require liming to improve productivity, it is unlikely that economic crops are being produced in soils so highly acid that micro-organisms could not function to degrade persistent residues of insecticides. The major problem at present is to learn what conditions are optimal for the development of pesticide-degrading micro-organisms and how such development can either be prevented (for residual control of pests) or encouraged (for destruction of persistent residues in the soil environment and especially in ground water).
REFERENCES A h m a d , N., Walgenback, D.D. and Sutter, G.R., 1979. Degradation rates of technical carbofuran and a granular formulation in four soils with k n o w n insecticide use history. Bull. Environ. Contam. Toxicol., 23: 572--574. Felsot, A., Maddox, J.V. and Bruce, W., 1981. Enhanced microbial degradation of carbofuran in soils with histories of Furadan use. Bull. Environ. Contain. Toxicol., 26: 781--788. Fuhremann, T.W. and Lichtenstein, E.P., 1980. A comparative study of persistence, movement, and metabolism of six carbon-14 insecticides in soils and plants. J. Agric. Food Chem., 28: 446--452. Getzin, L.W., 1973. Persistence and degradation of carbofuran in soil. Environ. Entomol., 2 : 461--467. Gorder, G.W., Dahm, P.A. and Tollefson, J.J., 1982. Carbofuran persistence in cornfield soils. J. Econ. Entomol., 75: 637--642. Greenhalgh, R. and Belanger, A., 1981. Persistence and uptake of carbofuran in a humic mesisol and the effects of drying and storing soil samples on residue levels. J. Agric. Food Chem., 29: 231--235. Miles, J.R.W. and Harris, C.R., 1981. A laboratory study of the persistence of carbofuran and its 3-hydroxy- and 3-keto-metabolites in sterile and natural mineral and organic soils. J. Environ. Sci. Health, B 16: 409--417. Read, D.C., 1976. Comparisons of residual toxicities of 24 registered or candidate pesticides applied to field microplots of soil by different methods, J. Econ. Entomol., 69: 429--437. Read, D.C., 1981a. Adultieidal and larvicidal toxicity of aldicarb when applied as a soil insecticide. J. Econ. Entomol., 74: 40--44. Read, D.C., 1981b. Toxicity of propoxur in wet and dry mineral soil to adults and larvae of the cabbage maggot, Hylemya brassicae (Diptera: Anthomiidae). Can. Entomol., 113: 1093--1100. Robinson, J.R. and Chapman, R.A., 1980. Determination of carbofuran residues in potato, onion, and turnip: comparative analyses using gas chromatography -- selected ion monitoring on high pressure liquid chromatography. J. Chromatogr., 193: 213-224.
57
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
MOVEMENT AND RESIDUAL TOXICITY OF CARBOFURAN IN ACID MINERAL SOIL*
D.C. READ ~ and S.O. GAUL 2
Agriculture Canada, Research Station, Charlottetown, Prince Edward Island CIA 7M8 (Canada) 2Agriculture Canada, Research Station, Kentville, Nova Scotia B4N 1,]5 (Canada) (Accepted 6 October 1982)
ABSTRACT Read, D.C. and Gaul, S.O., 1983. Movement and residual toxicity o f carbofuran in acid mineral soil. Agric. Ecosystems Environ., 9 : 57---68. In greenhouse miniplots with soil moisture moving upward from the subsoil, toxicants of carbofuran moved upward and became detectable at the soil surface in ca. 3, 7, and 28 days after 10% granules of the insecticide were banded 1, 3, and 10 cm, respectively, below the surface. The test soil had a pH of 5.2 and contained 2.9% organic matter. Toxic residues reaching the soil surface from a depth of 5--10 cm increased over a period o f 8--12 months and gradually decreased thereafter. Fifteen to 20 ppm in the surface soil was highly toxic to flies contacting the soil surface. A m o u n t s in excess o f 40 ppm in the surface layer caused knockdown in 10--15 and estimated death in less than 30 s. The LDs0 for cabbage maggot eggs on the surface of treated soil was 8.2 ppm and for larvae 0.86 ppm. Chemical analysis records correlated well with those o f bioassay tests. In field microplots, the addition o f water during rainstorms (giving a total of 50 m m ) demonstrated that toxic residues of carbofuran could be leached downward to at least 45 cm. However, under the normal weather conditions of 1980 and under abnormally dry conditions most o f the detectable residues remained near or above the point o f application. INTRODUCTION
Carbofuran has been used extensively for more than a decade to control soil and foliage insect pests of crops. Although several recent studies have been concerned with the influence o f micro-organisms on the rate o f degradation (see Felsot et al., 1981), and Getzin (1973) has observed that the c o m p o u n d degraded much more rapidly in alkaline than in highly acidic soils, little attention has been given to m o v e m e n t and persistence o f toxic residues in soil. Many o f the studies on the persistence of carbofuran have involved labora. tory tests conducted with technical or reference grade material thoroughly *Contribution No. 501, Agriculture Canada, Research Station, Charlottetown; and No. 1765, Agriculture Canada, Research Station, Kentville.
0167-8809/83/$03.00
© 1983 Elsevier Science Publishers B.V.
58 mixed with the soil and it is difficult to relate results of such tests with persistence or movement of residues of commercial granular formulation as applied for control of soil insects. The significance of this problem was clearly demonstrated by Ahmad et al. (1979) who found that technical carbofuran showed 50% degradation in 7 days in the same soil in which a 10% granular formulation degraded only 25--35% in 35 days. Fuhremann and Lichtenstein (1980) studied the persistence and movement of carbofuran and other compounds in soil, but t h e y too used labelled and pure chemicals mixed thoroughly with their soils. The present study was designed to determine the m o v e m e n t and persistence of commercial grade 10% granular carbofuran applied at different depths in acid mineral soils n o t previously treated with pesticides. M AT ER I ALS AND METHODS
The soil used in the microplot and miniplot tests comprised a mixture of a sandy soil and a fine sandy loam to provide a texture which minimized the problem o f baking and cracking after surface watering. The soil was collected in bulk lots (ca. 1000 kg during September of each year) from fields n o t previously treated with pesticides, screened to remove stones and plant roots, and thoroughly mixed. The mixture, containing 2.9% organic matter and 11% clay and having a pH of 5.2, was stored prior to testing during the fall, winter, and spring in open-top plastic bags in an unheated storage building. A 10% granular formulation of carbofuran was used in all of the experiments. The concentration applied to the soil was estimated at more than twice t h a t normally used for field applications using a subsurface band or seed furrow t r e a t m e n t for vegetable insect control. A commercial application of 3 kg AI ha -1 in a 10-cm wide subsurface band may be estimated to be roughly 46 ppm in the upper 5 cm of soil. The a m o u n t applied in most of these experiments was established at 100 ppm in the 5-cm depth. Higher and lower concentrations were also used for specific tests. All of the miniplot and microplot experiments were replicated at least three times or a series of miniplots of one t r e a t m e n t was established to allow testing of three replicates at interval times of sampling. Miniplot tests were conducted using 6-cm square standard plastic drain pipe cut into 15-cm lengths. For greenhouse studies, series of the square plastic miniplots were filled with the test soil and set up on a base o f 3 cm o f soil overlying 5 cm of sphagnum peat moss on a greenhouse bench. The insecticide was banded at the required depth below the soil surface in the miniplot. Fifteen cm diameter miniplots and the 6-cm square type were also used for testing the m o v e m e n t of subsurface spot applications of carbofuran. The temperature in the greenhouse c o m p a r t m e n t was maintained at 20 + 2°C. Sunlight intensity was restricted using 15-cm wide strips of styrofoam placed 15 cm apart and running north to south across the top and south side o f the c o m p a r t m e n t .
59 A field miniplot comprised a 45-cm depth of soil in three o f the 15-cm long sections of plastic pipe taped together with waterproof tape to facilitate separation of different depth layers at the time of sampling. Replicates of these miniplots were imbedded vertically in the soil with ca. 1 cm of the plastic protruding above the surrounding soft. Carbofuran was applied 3 cm below the soil surface. Testing for toxic residues at different depths in the soil involved sliding the block of soil from the 15-cm long sections o f the miniplots and separating 1 or 5-cm depth layers as required for analyses. The soil in the greenhouse miniplots received moisture from the underlying moistened soil-over-moss base or b y sprinkling water over the surface. In field tests, one set o f three replicated miniplots was exposed to normal weather conditions while additional water was added to a second replicated set to give 'above normal' rainfall. For the field microplot tests, soil was encompassed within a 3
60 maggot. For the adulticide tests 300--500 flies were released into cages conraining a series of replicated miniplots in which insecticide had been banded at different depths below the soil surface. The number of flies killed due to contact with the soil surface gave an indication of the levels of toxicant reaching the surface within a given time from insecticide applications at different depths. The p h e n o m e n o n of the high p o t e n c y of certain insecticides in killing flies in contact with the surface of treated soil, from an insecticide either mixed into the surface soil or applied at various depths below the surface, is discussed in detail in previous papers on aldicarb and p r o p o x u r (Read, 1981a, b). Records of tests on subdivided layers of soil removed from the miniplots at different times after toxicants became detectable at the soil surface gave an estimate of the levels of toxic residues at different levels above and below the point of application. Ovicidal and larvicidal toxicity was measured by placing 100 eggs (ca. 36 h prior to hatching} in 5-cm diameter petri dishes containing 5 g of a sample of test soil. A small rectangular slice of rutabaga was placed in the centre o f each petri dish between t w o lots of 50 eggs as food for surviving larvae. Records on egg and larval mortality and tunnels in rutabaga slices were taken 24 h after egg hatch. Initial toxicity tests with several insecticides mixed with soil showed that organophosphorus and carbamate materials at 0.2--15 p p m caused ca. 5--99% mortality of larvae, while levels of 10--80 p p m caused ca. 5--99% egg mortality. In situations where a treatment caused 100% kill of eggs, tests were repeated with serial dilutions of the test soil mixed with untreated soil to enable calculation of the approximate levels of toxic residues in the treated sample. Confirmation of levels of residual toxicants of carbofuran in soil from selected tests was made by comparing the results of bioassay tests with those of chemical analysis. The portions of the samples from the greenhouse miniplots or field microplots to be tested for chemical analysis were air-dried and stored a t - 2 0 ° C prior to extraction and analysis. Chemical analysis involved the gas--liquid chromatographic (GLC) determination of carbofuran from soil extracts with an alkali flame ionization detector. Extraction of soils (50.0 g or the total sample) followed the m e t h o d of Robinson and Chapman {1980}. The chloroform extracts were evaporated to a small volume, made up to volume (15 ml} in ethyl acetate, and injected (2 ~1} directly. To increase detector stability, clean-up of the extract using a fast wool plugged Pasteur pipette column containing 5 cm Florisil t o p p e d with 1 cm anhydrous sodium sulfate was carried out. One ml of soil extract was added to the pre-rinsed column followed by 10 ml ethyl acetate. The eluant was reconcentrated to 1 ml before GLC injection. A Tracor Microtak 220 GLC was used equipped with a Model 702 Nitrogen-Phosphorus detector. The column was glass (2 mm id × 60 cm) packed with 1.5% OV-17 + 1.95% OV-210 on Chromosorb WHP, 80--100 mesh. Operating conditions were: oven temperature, 143 ° C; inlet temperature, 200 ° C; d e t e c t o r temperature, 285°C; carrier gas (helium} flow rate, 80 ml min -~ ;
61 hydrogen 2.2 ml min-1; compressed air, 60 psig at tank; electrometer attenuation, 1 X 8 or 1 × 16. O u t p u t was recorded on a Westronics/mv f.s.d, recorded run at a chart speed of 1.27 cm min -1 . Alternate injections of standard, sample, and appropriate standard were made and the ca~bofuran concentration determined on the basis of peak heights. The limit of detection with the operating conditions used was 0.04 p p m and recovery from spiked samples with 0.5--1.5 p p m ranged from 85 to 102%. Results are reported only for carbofuran. No 3-hydroxy carbofuran was detected in any of the samples and tests were n o t conducted for residues of the 3-keto metabolite. Miles and Harris (1981) demonstrated that the two metabolites comprised a relatively small percentage of the total carbofuran residues present in the soil. RESULTS
The data in Table I show the adulticidal toxicities o f carbofuran compared with those of six other insecticides and demonstrate that the adulticidal c o m p o u n d s were continually moving toward the surface of the soil in the greenhouse miniplots. During the period of peak toxicity for a c o m p o u n d , as at 11--20 days for the 1-cm depth o f aldicarb, k n o c k d o w n o f flies contacting the soil surface occurred in 10--15 s and detectable twitching of legs and wings had ceased in less than 30 s. Other adulticide bioassay tests with applications at depths of 5 and 10 cm demonstrated that insecticides applied at the lower depths did n o t reach the high concentrations at the soil surface as attained from 1 to 3 cm depths. However, toxic residues persisted longer. Base-line toxicity data for laboratory tests on carbofuran as an ovicide and larvicide are presented in Table II. These records show that 2--40 p p m caused ca. 1--100% mortality of eggs and concentrations of 0.2--3 ppm resulted in 0--100% mortality o f test larvae. Records of toxicity tests c o n d u c t e d at different times after soil application of carbofuran in greenhouse tests were compared with the data in Table II to obtain estimates of toxic residues remaining in the soil. The results presented in Table III demonstrate the correlation between bioassay and chemical analysis. These data show the relative amounts of toxic residues persisting at different levels in the soil 75 days after carbofuran was banded at a depth of 10 cm. The highest levels had become concentrated near the soil surface and residues near the point of application were greatly reduced. After 200 days, all detectable residues were concentrated near the surface and no toxicant could be detected at or near the original point of application. Records of the greenhouse bioassay tests on long-term persistence (Table IV) indicate that toxic residues were continually moving toward, and concentrating at, the surface. Results of a series of experiments (data n o t included), with tests c o n d u c t e d after 25, 50, 100, and 150 days, supported the data in Tables III and IV on the upward m o v e m e n t of toxic residues. Results of chemical analysis averaged for the nine layers of soil for which
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63 TABLE II Initial percentage mortalities o f cabbage maggot eggs and larvae caused by different concentrations of carbofuran in acid mineral soil and tested in 5-g samples of treated soil in 5-cm diameter petri dishes a Insecticide in soil (ppm)
50 40 30 20 15 10 5 2 1 0.8 0.4 0.2 Untreated
Mortality b (%)
Tunnels in rutabaga
Eggs
Larvae
(slices by surviving larvae)
100 100 98 82 77 61 24 14 8 5 5 5 5 5
100 100 100 100 100 100 100 100 87 62 27 14 0 0
0 0 0 0 0 0 0 0 13 34 Moderate Extensive Extensive Extensive
aAverages for four tests conducted at different times (with different lots of eggs) with two replications of each test. bEggs incubated on treated soil for at least 36 h prior to hatching. Larvae from eggs placed on treated soil ca. 2--6 h prior to hatching.
TABLE
III
Carbofuran residues at different levels in the soil 75 days after the insecticide was banded at a depth of 10 cm in miniplots at 20 ppm in the total volume of soil Depth layer of soil (cm)
0--1 1--2 2--4 4--6.5 6.5--9 9--11 b 11--12 12--13 13--15
Carbofuran residues (ppm) a Chemical analysis
Bioassay estimates
27.68 7.95 5.38 2.13 2.98 6.34 ND 0.79 0.23
32 9 6.3 1.5 4.5 7.5 ND 1.2 ND
aND = non-detectable residues by chemical analysis -- less than 0.04 ppm; by bioassay -no larval mortality. bDepth layer containing original application, estimated to have contained 150 ppm carbofuran at the time o f application.
64 TABLE IV Percentage mortalities of cabbage maggot eggs and larvae and estimated residues in ppm of carbofuran 200 days after insecticide was banded at a depth of 10 cm in 6-cm square miniplots to give 30 ppm in total volume of soil
Soil depth tested (cm)
0--1 1--2 2--4 4--6 6--8 8--9 9--11 b 11--13 13--15 Untreated
Mortality (%) Eggs
Larvae
99 36 17 11 8 3 5 4 5 4
100 100 92 43 7 0 0 0 0 0
Estimated level of residues (ppm) a
30 6 2.5 1 0.5 ND ND ND ND ND
aND = none detectable. Data determined from a Log--Probit curve prepared from data used for Table II. b Location of original band of insecticide (at 10-cm depth). TABLE V Upward and horizontal movement in soil from a coin-sized (1.25-cm diameter) con-centrated application of 10% carbofuran granules placed at a depth of 7.5 cm below the surface and in the centre of 15-cm diameter miniplots and analysed after 55 days a
Depth of soil sample (cm)
0--2 2--4 4--6 6--8 8--10
Concentration of carbofuran (ppm) b 'Centre'
'Adjoining'
'Outer'
12.88 5.72 9.96 2513.90 3.52
6.06 1.41 1.23 1.13 ND
1.28 0.71 ND ND ND
aChemical analysis data. Bioassay estimates correlated closely with these data. bResidues at different depths and from 2.5-cm diameter cores of soil taken in the 'centre' (directly over the original concentrated application), two 'adjoining' cores, and four ' o u t e r ' cores located around the centre core and 0.5 and 3.5 cm, respectively, away from the outer edge of the centre core. d a t a o n t h e d i f f e r e n t l a y e r s is g i v e n i n T a b l e I I I s h o w e d a r e s i d u e o f 5 . 8 4 p p m . T h i s r e p r e s e n t s t h e r e s i d u e i n t h e t o t a l v o l u m e o f soil a n d i n d i c a t e s a d e g r a d a t i o n o f ca. 7 0 % d u r i n g t h e 7 5 - d a y t e s t . In greenhouse experiments using 15-cm diameter and 15-cm deep miniplots in a d d i t i o n to the 6-cm square m i n i p l o t s placed over t h e soil-moss base,
65
but with a concentrated (1.25-cm diameter) spot application of 10% carbofuran granules placed in the centre and 7.5 cm below the soil surface, the toxicant moved directly upward in the centre of the miniplots and horizontal movement appeared to be limited. The results of one replicated experiment, which was sampled and analysed after 55 days, are presented in Table V. Bioassay tests conducted with similar central subsurface spot applications and tested at intervals over a period of 2.5 years showed similar trends of TABLE VI Residues (ppm) of carbofuran in different depth layers of soil 90 and 130 days after soil treatment at a depth of 2.5 cm in field microplots and miniplots receiving 'below normal', 'normal', and 'above normal' amounts of rainfall during the growing season as measured by chemical analysis and bioassay a Depth of soil layers (cm)
Rainfall regimen for first 90 days in microplots Below normal (155 mm)
Normal (265 ram)
Above normal (310 mm)
Residues in triplet layer microplots within aluminum frames (12 × 25 cm)
0--5 5--10
10--15
Chemical
Bioassay
Chemical
Bioassay
Chemical
Bioassay
68.25 0.57 0.56
76 Trace Trace
73.0 1.91 0.98
61.4 3.4 2.6
----
53.5 6.5 1.5
Rainfall regimen for 130 days in microplots b
0--5 5--10 10--15 15--20 20--25
Below normal (210 ram)
Normal (350 ram)
Above normal (435 mm)
48.13 0.87 0.73 0.17 ND
48.49 3.38 0.85 ---
47.14 0.81 0.26 0.14 0.58
43.5 1.5 0.4 0.6 0.8
20.99 3.77 1.22 0.82 0.78 0.93 0.88 0.78 0.29
18.5 4.2 1.0 0.8 0.8 1.1 0.6 0.4 ND
59 1.4 ND ND ND
55.0 4.5 1.5 ND ND
Residues in 45-cm deep miniplot column after 130 days 0--5 5--10 10--15 15--20 20--25 25--30 30--35 35--40 40--45
35.20 4.26 0.13 0.45 0.15 0.37 Trace ND ND
38.45 4.8 0.8 0.5 0.5 ND 'N D ND ND
aOriginal application in the 0--5 cm depth was 115 ppm in the microplots and 100 ppm in the miniplots. bSamples at 15--20 and 20--25 cm depths were core samples from the subsoil under the microplots.
66
upward movement, limited horizontal spread, and also of extreme persistence of the carbofuran in the concentrated central core containing the original application. Two years after applying such a spot application (calculated at 3000 p p m in 10 g of soil immediately surrounding the carbofuran granules and situated between 7 and 8 cm below the soil surface), toxic residues of ca. 600 p p m still remained in the central 10-g core. Residues in the 0--6-cm depths directly above the granules remained at high levels for 10--14 months, especially near the surface, and then gradually decreased. Data from field microplot tests c o n d u c t e d during the growing season (midJune to late-October) indicated that 'below normal' rainfall conditions resulted in a slightly faster rate of degradation of carbofuran than where the surface soil was fully exposed to normal rainfall (Table VI). Nevertheless, most o f the residues in the 'normal' and 'below normal' rainfall microplots remained in the 0--5-cm layer in which the carbofuran had been applied. However, additional water added to the 'above normal' microplots increased d o w n w a r d movement of toxic residues. This downward m o v e m e n t was further demonstrated in the 45-cm deep field miniplots where residues were found at all depth levels. It is significant to note that an apparent state of equilibrium occurred between 15 and 40 cm below the soil surface, with all five levels showing residues of 0.85 + 0.07. DISCUSSION
The results of these greenhouse and field experiments demonstrate that carbofuran is quite mobile in soil and that subsurface band or furrow applications, in general, tend to move upward and become concentrated near the soil surface. Since this trend is correlated with movement o f soil moisture, it is assumed that subsurface insecticide applications in ridged rows in a field would always tend to move upward because excessive rainwater from a heavy rainfall would run o f f the sides of the rows. This could explain w h y a single pre-planting application of relatively non-persistent insecticides, such as carbofuran or propoxur, has given all-season protection of rutabagas against r o o t maggot infestations. Such persistence was particularly apparent during a relatively dry growing season when p r o p o x u r gave slightly better r o o t maggot control than carbofuran. By comparison, p r o p o x u r gave no apparent protection during abnormally wet seasons (Read, 1976, 1981b). With ground level plantings, and particularly where the packing wheel behind the seeder could make a shallow trench over the insecticide and seed, the records in Table VI suggest that heavy rainfall could leach carbofuran residues d o w n below the point of attack by insects. Therefore, pre-planting seed-furrow or subsurface band treatments would be expected to give poor residual control during abnormally wet growing seasons. Several articles have recently been published on enhanced degradation of carbofuran in soil containing adaptive carbofuran
67 that the ramifications of enhanced microbial degradation in 'microbe' soil have intentionally been excluded from this report, especially the influence under high moisture or essentially anaerobic conditions (see Gorder et al. (1982) for references). Our recent studies have shown that the carbamate insecticide, carbofuran, added to 'carbofuran microbe' soil and the organophosphorus insecticide, fensulfothion, added to 'fensulfothion microbe' soil (resulting from repeated field applications of the insecticide to the same soil) degrade much more rapidly than in previously untreated or 'normal' soil and under normal field moisture conditions. The use of 'microbe' soils during this study with carbofuran caused variations in persistence and increased the complexity of the basic study on movement. Nevertheless, even in soils k n o w n to contain these micro-organisms, the pattern of m o v e m e n t of toxicant was the same as described in this paper. The most significant difference was that the persistence of toxicants varied greatly at different levels in the soil profile and was retarded in air dry soil. In tests with 'microbe' soil, set up in both the 6-cm square and the 15-cm diameter miniplots, the initial m o v e m e n t from a subsurface band application followed precisely the same trend toward the surface as for the 'normal' soil (data given in Table III). When banded at a depth of 7.5 cm, carbofuran initially moved upward, as in the 'normal' soil, and became concentrated near the surface. However, residues disappeared more quickly from the region near the point of application and after 50--60 days trace amounts of toxicant could be detected only in the t o p few millimeters of soil. The same initial upward m o v e m e n t occurred when spot concentrations of carbofuran granules were placed 7.5 cm deep and in the centre of miniplots o f 'microbe' soil. High concentrations of the insecticide were detected at the surface in 15--20 days. However, after ca. 30 days all traces of residues had disappeared from all of the upper layers of soil and after 50--60 days toxicant could only be detected at and just above the point where the spot concentration had been applied. Breakdown was more rapid in the 15-cm diameter miniplots than in the 6-cm square miniplots and it was assumed that the adaptive degrading microorganisms were moving inward from the sides of the miniplots and breaking d o w n the carbofuran as it was moving upward from the centre spot application. The micro-organisms were apparently unable to rapidly degrade the spot application (ca. 50 000 p p m in the 1.25-cm diameter core). Subsequent tests have confirmed the above observations and results will be reported in detail later. It is important to note at this time that even the enhanced microbial degradation was essentially eliminated in highly acid soil (below a pH of ca. 4.5). Hence, both chemical breakdown, as demonstrated by Getzin (1973), and microbial degradation, normal or enhanced, are greatly retarded in.highly acid soils. For soils with pH values of 4.3 and 6.0, Getzin (1973) found carbofuran to have a relatively long half-life (more than 50 weeks in highly acid soil). In our 'microbe' soil with pH values in the range of 6.4 d o w n to 5.1, adaptive micro-organisms could degrade 1000 ppm of carbofuran in less
68
than 30 days, but were essentially inactive at pH levels of 4.5 or lower. Since most highly acidic mineral soils require liming to improve productivity, it is unlikely that economic crops are being produced in soils so highly acid that micro-organisms could not function to degrade persistent residues of insecticides. The major problem at present is to learn what conditions are optimal for the development of pesticide-degrading micro-organisms and how such development can either be prevented (for residual control of pests) or encouraged (for destruction of persistent residues in the soil environment and especially in ground water).
REFERENCES A h m a d , N., Walgenback, D.D. and Sutter, G.R., 1979. Degradation rates of technical carbofuran and a granular formulation in four soils with k n o w n insecticide use history. Bull. Environ. Contam. Toxicol., 23: 572--574. Felsot, A., Maddox, J.V. and Bruce, W., 1981. Enhanced microbial degradation of carbofuran in soils with histories of Furadan use. Bull. Environ. Contain. Toxicol., 26: 781--788. Fuhremann, T.W. and Lichtenstein, E.P., 1980. A comparative study of persistence, movement, and metabolism of six carbon-14 insecticides in soils and plants. J. Agric. Food Chem., 28: 446--452. Getzin, L.W., 1973. Persistence and degradation of carbofuran in soil. Environ. Entomol., 2 : 461--467. Gorder, G.W., Dahm, P.A. and Tollefson, J.J., 1982. Carbofuran persistence in cornfield soils. J. Econ. Entomol., 75: 637--642. Greenhalgh, R. and Belanger, A., 1981. Persistence and uptake of carbofuran in a humic mesisol and the effects of drying and storing soil samples on residue levels. J. Agric. Food Chem., 29: 231--235. Miles, J.R.W. and Harris, C.R., 1981. A laboratory study of the persistence of carbofuran and its 3-hydroxy- and 3-keto-metabolites in sterile and natural mineral and organic soils. J. Environ. Sci. Health, B 16: 409--417. Read, D.C., 1976. Comparisons of residual toxicities of 24 registered or candidate pesticides applied to field microplots of soil by different methods, J. Econ. Entomol., 69: 429--437. Read, D.C., 1981a. Adultieidal and larvicidal toxicity of aldicarb when applied as a soil insecticide. J. Econ. Entomol., 74: 40--44. Read, D.C., 1981b. Toxicity of propoxur in wet and dry mineral soil to adults and larvae of the cabbage maggot, Hylemya brassicae (Diptera: Anthomiidae). Can. Entomol., 113: 1093--1100. Robinson, J.R. and Chapman, R.A., 1980. Determination of carbofuran residues in potato, onion, and turnip: comparative analyses using gas chromatography -- selected ion monitoring on high pressure liquid chromatography. J. Chromatogr., 193: 213-224.