Are Microorganisms Getting To Your Pesticide ??
H.
D. Skipper1,
R. O. Ankumah2,
N. Gaskin2,
D. D. Brown2,
J. Hamilton2,
L. W. Grimes1,
J. H. Kim1,
and T. L. Lalande1.
1Clemson
Univ., Clemson, SC and 2Tuskegee
Univ., Tuskegee, AL.
Introduction
The
purpose of this report is to discuss the environmental fate of Nemacur™.
This
pesticide is also known by the common chemical name of fenamiphos and is used to
control certain nematodes and insects in turf.
We will cover the role of microorganisms in the persistence of fenamiphos
in golf greens and potential impact on performance failures after repeated uses
of fenamiphos. Key terms related to
microbial degradation will be defined and selected graphs will be used to
illustrate half-lives.
BIODEGRADATION
is the transformation of complex organic chemicals by microorganisms into simple
end products such as CO2
and H20. Plants may also degrade or detoxify pesticides, but for the
purpose of this article, we will focus on biodegradation by microorganisms.
To assist with understanding this report, a series of definition are
provided.
·
ADAPTED
MICROORGANISMS can use a single pesticide for growth.
·
CROSS-ADAPTED
MICROORGANISMS can use more than one pesticide for growth.
These pesticides are generally similar in structure and often belong to
the same chemical class.
·
A HISTORY
SOIL has received repeated applications of a specific pesticide.
·
A
NON-HISTORY SOIL has no known history of being treated with a specific
pesticide.
·
A PROBLEM
SOIL indicates a pesticide does not control the pest as expected and there is no
performance from the pesticide in question.
This lack of performance is often linked to a history of repeated
applications.
·
ENHANCED
may be defined as accelerated, greater, faster, or elevated.
·
BIODEGRADATION
indicates a biological agent such as a fungus or bacteria is responsible for the
degradation.
·
Thus, ENHANCED
BIODEGRADATION is a more rapid breakdown or detoxification of a pesticide
than normal by biological agents in the soil/root mix.
Thus,
when a golf course superintendent or turf farm manager notices that fenamiphos
does not control nematodes as expected, enhanced biodegradation could be one of
the possible explanations. However,
there are other potential causes to explain performance failures with
pesticides.
Why pesticides
don’t work:
·
ENVIRONMENTAL
FACTORS - extremes in temperature,
moisture, relative humidity, or pH may deactivate a pesticide.
· MANAGEMENT FACTORS - improper selection of pesticide or inaccurate timing or calibration or application of a pesticide may produce poor results.
·
RESISTANT
PESTS - after repeated applications
of a certain pesticide, some pest may become resistant to future applications of
that pesticide and a new pesticide or alternative technique is required for
control.
·
ENHANCED
BIODEGRADATION – after repeated applications of some pesticides, soil
microorganisms detoxify the pesticides before they can control the pest.
These microorganisms become adapted to using the pesticide as a carbon or
energy source and they can grow on it.
If
the environmental and management factors and resistant pests can be eliminated,
then research efforts can be concentrated on enhanced biodegradation as a
causative factor. Since maximum
efforts are made to grow healthy, vigorous turf, conditions are also favorable
for microbial growth.
Potential risk
for enhanced biodegradation in turf is high because:
·
OPTIMAL
CONDITIONS - moisture,
nutrients, organic matter, root exudates and secretions exist for microorganisms
to use.
·
MONOCULTURE
- greens and fairways are
continuously in grass.
·
INTENSIVE
USE OF PESTICIDES - many pesticides
are used on golf courses and often more than once per year.
Pesticide
Failure---The Problem: When a pesticide is degraded by microorganisms in a soil or green
mixture at an accelerated rate, the persistence or half-life of the pesticide is
too short to provide acceptable control of the pest and the result is considered
a performance failure. The
phenomenon is known as enhanced biodegradation in problem soils and is
associated with the repeated or continuous use of certain pesticides.
As illustrated in Figure 1, the half-life (50% of the pesticide remains)
of the pesticide is 25 days in the non-problem soil.
However, in the problem soil, the half-life or t1/2
is only 8 days.
In a similar manner, after repeated applications the half-life is reduced
from 20 days with the first application to 10 days with the second application
and subsequently to 5 days with the third application of a specific pesticide.
Thus, the pesticide is detoxified by the microorganisms before the
chemical can control the pest.
Enhanced
biodegradation of pesticides is responsible for loss of efficacy and ultimately
for the performance failures of certain pesticides in agronomic and vegetable
crops in many regions of the USA (Racke and Coats, 1990; Skipper, 1990).
However, there are only limited studies to ascertain the role or impact
of enhanced biodegradation of pesticides in golf greens, fairways, or turf
farms.
Enhanced
biodegradation has been reported for a number of classes of pesticides including
the organophosphate pesticides (Racke and Coats, 1990). Fenamiphos
[Nemacur:
ethyl 3-methyl-4-(methylthio) phenyl (1-methylethyl) phosphoamidate] is an
organophosphorus nematicide used to control soil borne nematodes and some
insects. Enhanced degradation of
fenamiphos on soils with previous application history has been reported by
several investigators (Ou and Rao, 1986; Ou et al., 1991, Davis et. al 1993; Ou
et al 1993; Ou et al., 1994; Chung and Ou, 1996; Smelt et al., 1996).
Ou (1991) reported enhanced
biodegradation of fenamiphos in soil collected from a potato field that had
received a single application of fenamiphos two years after treatment.
The
effect of enhancement has been observed to last as long as 3 years after initial
application (Ou, 1991). Long term
continuous application of the fenamiphos on turf was also found to result in
rapid degradation of the pesticide and its metabolites (Ou et al., 1993; Ou et
al., 1994). The increased
degradation of fenamiphos and its metabolites in soils with history of
application has been attributed to microbial degradation of the parent compound
and its metabolites (Ou et al., 1994, Ou and Thomas, 1994; Chung and Ou, 1996;
Skipper et al., 1998). Smelt et al.
(1996) reported a strong linkage between soil pH and the development of enhanced
biodegradation for certain pesticides in the Netherlands. To protect
environmental quality and to retain acceptable levels of pest management for
golf courses, the influence of environmental factors and use of pesticides on
enhanced biodegradation must be determined.
Objective
Determine if enhanced biodegradation is responsible for performance failures of fenamiphos used on golf courses to control nematodes or other pests in golf greens in South Carolina.
Sites:
Green mix samples were collected from Callawassie, Dunes West, Secession, Stono
Ferry, and Wild Dunes Golf Courses in the Charleston-Kiawah Island-Beaufort area
of South Carolina. According to
golf course superintendents, these courses had experienced poor performance by
fenamiphos in recent years and the lack of nematode control reduced the growth
and vigor of the greens. Some
members of the turf industry suspected the performance failures were linked to
enhanced biodegradation by soil microorganisms.
Control samples with no history of fenamiphos use were collected from
Cougar Point Golf Course.
Experimental
processes:
50 g of dry weight mix from each green were placed in 250 mL soil biometer
flasks (Bartha and Pramer, 1965; Skipper et al., 1986).
Each mix was brought to approximately field capacity with the addition of
the respective treatment solutions, water, and mixed well.
Duplicate samples were incubated at 23 to 25oC
for a period of 35 days. All
treatments were expressed on a soil dry weight basis and the final concentration
of fenamiphos was 10 ppmw from radioactive (14C-labeled) and technical fenamiphos.
Determination
of fenamiphos biodegradation: As the fenamiphos was degraded by microorganisms,
the radioactive carbon would be evolved/respired as carbon dioxide (14CO2)
that was captured in 10 mL of 0.5 M
sodium hydroxide. The sodium
hydroxide trapping solution was replaced with the same volume of fresh solution
on a Monday-Wednesday-Friday schedule. An
aliquot of 0.5 mL of the NaOH solution containing 14CO2
was added to 4.5 mL Ecolume liquid scintillation fluid, vortexed, and counted
for 5 min on a Packard Tri-Carb 1900CA Liquid Scintillation Counter.
Degradation of fenamiphos was obtained for each green mix by calculating
the percentage of degradation by dividing the radioactivity (DPM) of the
captured 14CO2
by the total input radioactivity (total DPM).
The biodegradation data are presented in Figures 2 and 3. In each figure, the control data are from the Cougar Point Golf Course that had no history of fenamiphos use since it was reconstructed in 1996. Sodium azide was an effective microbial inhibitor in selected treatments and reduced the degradation of fenamiphos to approximately 10% at day 35 (data not shown).
At day 7, there were no differences between the Secession greens (Figure 2) previously treated with fenamiphos and the control samples (non-history) in the biodegradation of fenamiphos. However, at day 14 through day 35, the fenamiphos-history greens had a significantly higher degree of biodegradation than the control green (~60% versus 30%). From day 27 to day 35, there was a very rapid increase in biodegradation of fenamiphos in the control mix to suggest that exposure to one application of fenamiphos was sufficient to induce enhanced biodegradation of fenamiphos as noted for some carbamothioate herbicides (McCusker et al., 1988). A similar pattern was noted for the Dunes West greens (data not shown).
The Callawassie greens were comparable at day 7 (Figure 3); however, by day 15 through day 35, the Number 2 Green (2G) was significantly more active than 4G. Thus, even at a single golf course, the degree of enhanced biodegradation varied from green to green. Both greens were significantly more active than the control in the biodegradation of fenamiphos.
With 70 to 80% degradation at day 35, the Stono Ferry greens (data not shown) had the highest rate of decomposition of fenamiphos in this study. By day 14, all greens were significantly more active than the control green. For the Wild Dunes greens, a sample from the collar of Number 1 Green exhibited approximately 25% degradation by day 7 and was significantly more active than the other samples (data not shown). At this location, both greens and associated collars that had received fenamiphos treatments were significantly more active than the control sample.
Summary
Based on results from five golf courses with performance failures by fenamiphos in South Carolina, enhanced biodegradation was a key, and most likely, the dominant factor. When feasible, superintendents and turf farm managers should rotate fenamiphos with other chemicals or alternative control measures to control nematodes or insects on their golf courses or turf farms. In the case of poor performance by fenamiphos after an initial application, a thorough evaluation of environmental and management factors and potential pest resistance should be made first to rule them out as the reason(s) for poor control. If the causative factor is judged to be enhanced biodegradation, a second or third application to achieve control would most likely be ineffective and serve only to enlarge the microbial population responsible for biodegradation of fenamiphos.
Selected
References
Bartha, R., and D. Pramer. 1965. Features of a flask and method for measuring the persistence and biological effects of pesticides in soil. Soil Sci. 100: 68-70.
Chung, K.-Y., and L.-T. Ou. 1996. Degradation of fenamiphos sulfoxide and fenamiphos sulfone in soil with a history of continuous applications of fenamiphos. Arch. Environ. Contam. & Toxicol. 30: 452-458.
Davis R., A. W. Johnson, and R. O. Wauchoe. 1993. Accelerated degradation of fenamiphos and its metabolites in soil previously treated with fenamiphos. J. Nematol. 25: 679-685.