INSECTICIDE MIXTURE FORMULATIONS:
TOXICOLOGICAL ASPECTS AND PERSPECTIVES[1]
by:
Rudy C. Tarumingkeng, PhD[2]
<click on the footnote number
to see the footnote>
With the increasing awareness to the environmental
problems, increasingly pesticides are being
applied at reduced rates. Industries introducing new compounds, or extending
the commercial life of existing compounds, and are looking to the introduction
of mixed chemicals for pest management. In this regard, extra care should be
taken since the effect of pesticide mixtures are considered more toxic
than their individual components when used singly, and their tendency to
develop resistance to pests.
However, fast development of
pesticide technology, more appropriate use of pesticides and more improved IPM
methods, should render saver use of the mixtures but these still need much
exploration and research work. The latter should address the strengths and weaknesses of pesticide
mixture, do such mixtures provide the opportunity of overcoming resistance, improving
the biological spectrum or reducing the overall dose of chemicals applied and
how are such mixtures screened for synergy.
As with the use all toxic chemicals, the main
problem concerning the use of pesticides as well as their mixtures is of course
their potential to cause adverse effects to human health. Besides, problem
related to the development of pest resistance and the threat to the environment
and the non-target organisms which may adversely affect biodiversity of ecosystems,
are becoming increasingly important in considering the development of reduced
risk pesticides.
Perhaps the best filter for
a reduced risk pesticide is EPA's Pesticide Regulation[3]
as summarized in the factors below (in
descending order):
·
human health effects: very low mammalian toxicity,
toxicity generally lower than alternatives (10-100X), displaces chemicals that
pose potential human health concerns [e.g., organophosphates (OPs), probable carcinogens (B2s)], reduces
exposure to mixers, loaders, applicators and reentry workers.
·
non-target organism effects (birds): very low toxicity
to birds, very low toxicity to honeybees, significantly less toxicity/risk to
birds than alternatives, not harmful to beneficial insects, highly selective
pest impacts.
·
non-target organism effects (fish): very low toxicity to
fish, less toxicity/risk to fish than alternatives, potential toxicity/risk to
fish mitigatable, similar toxicity to fish as alternatives but significantly
less exposure.
·
groundwater (GW): low potential for GW contamination,
low drift and runoff potential, runoff mitigatable.
·
lower use rates than alternatives, fewer applications
·
low pest resistance potential (i.e., new mode of action)
·
highly compatible with IPM
·
efficacy.
Those
factors that most significantly contributed to an unacceptable decision by EPA
are summarized below in descending order:
·
human health effects: inadequate/inappropriate
comparisons with alternatives, inadequate documentation of effects, human
health risk reduction case weak, risk reduction case inadequate when compared
to alternatives
·
non-target organism effects (birds and fish): toxic to
birds, toxic to fish, risk reduction case inadequate when compared to
alternatives
·
potential GW problems
·
unlikely to displace higher risk alternatives
·
lack of efficacy data
·
phytotoxicity.
Toxicologically, pesticide formulation with more
than one active component is considered the same as mixtures of chemicals. The
effect of two or more chemicals applied simultaneously will produce a response
that may be simple additive of their individual responses or may be greater or
less than that expected by addition of their individual responses.
An additive effects is the
situation in which the combined effect of two chemicals is equal to the sum of
the effect of each component given alone (1 + 3 = 4). This is the most commonly
observed when applying mixtures of insecticides such as two organic phosphate
insecticides given together.
A synergistic effect is the
situation in which the combined effect of two chemicals is much greater than
the sum of effects of each component given alone (1 + 3 >>> 4). This effect is applied in
the use of piperonylbutoxides as synergist for pyrethroids, which could
increase the latter toxicity around a tenfold. Potentiation is the situation
when one substance does not have a toxic effect but when added to a toxic
chemical it makes the latter much more toxic
( 0 + 1 >>> 1). However, it is common in the area of pesticides
to consider effect of potentiation as the same as synergism.
Antagonistic effect is the
situation in which two chemicals when administered together interfere with each
other's actions or one interfere with the action of the other (2 + 3 > 5). Antagonistic effects are
useful in toxicology as they serve as basis of antidotes[4].
Since both additive and
synergistic effects produce increased toxicity, the simple conclusion from
applying pesticide mixture to pest population is that lethal action is faster
and high mortality of pest population is obtained.
Since the early years of the
present century pest resistance to pesticides has occurred in the San Jose
scale since the use of lime sulfur spray, and in the California Red scale
selected by hydrogen cyanide. Insecticide resistance began to receive attention
only after World War II when DDT failed to control resistant strain of the
housefly in Sweden and Denmark, the mosquito in Italy and the bedbugs in
Hawaii. Due to this general trend of pesticide resistance development, today it
is more relevant to consider the pest susceptibility than the pest resistance
to pesticides of the major pests of agriculture and public health. With the
invention of new pesticides and the widening scale of their application, cases
of pesticide resistance have continued to develop at an exponential rate[5].
A classic example of
insecticide resistance is demonstrated by the mechanisms of conversion of DDT
to the nontoxic DDE by the enzyme DDT-ase and the altered cholynesterase that
is not readily inactivated by the phosphate organic insecticides. These
detoxication traits or other means of survival that cause resistance to the pest
organism are developed as a result of the natural selection of pre-adaptive
mutants as genetic factors that controlled such mechanisms. They may be present
in a very low frequency in the pre-pesticide treatment population, but with
intensive and repeated pesticide treatments, the resistant gene frequency is
capable to increase to 90% in just a couple of years5.
In cross-resistance the
problem becomes more serious and extends to other pesticides within the class
of the pesticide selected for, when there is either common de-toxication system
or target-site insensitivity. This is the case of lindane-resistant mosquitoes
that are also non-susceptible to dieldrin (both chlorinated hydrocarbons), and
the DDT-resistant houseflies which are non-susceptible to methoxychlor.
Far more serious resistance
problem occur in multiple-resistance in which pesticides with differing modes
of action and different detoxication
pathways5. Insect pests such as the housefly, the mosquito Cules pipiens, and the tobacco budworm Heliothis virescens have developed
multiple-resistance to organochlorines, organophosphates, carbamates and
pyrethroids.
Although the gene frequency
of a specific resistant allele may decrease after the pesticide application
discontinues the residual inheritance persists in the genome thus causes the
strain to regain as soon as the pesticide is reapplied. This happens because
once selected for, resistant genes possess limitless persistence in wild pest
populations and this lead to resistance stability.
Based on their laboratory
and field experiment as well as basic research on the resistance management in
cotton bollworm, Helicoverpa armigera in
China, Jin Liangshen et al.
(1994)9
suggest the following guidelines regarding the proper use and mixtures of
pesticides:
1.
Pest
populations to be controlled should be susceptible or have low levels of
resistance to each pesticide used in the mixture. Under such conditions, the
frequency of multiple resistance genotypes in the population may be maintained
at lower levels.
2.
Each
pesticide of a mixture should exhibit no cross-resistance to populations of
resistant pest concerned.
3.
The
mixtures selected should have significant synergism to reduce the selection
pressure of pesticide to pest, and delay resistance development.
4.
In
formulating the mixture, each candidate pesticide component to be used for
mixtures should have lower mammalian in order to decrease overall mammalian
toxicity of mixture. Frequently mixtures with higher toxicity to pest produce
increasing toxicity to mammals.
5.
Insecticide
mixtures should be considered as one of the tactics for resistance management
strategy. If any pesticide is used extensively, there will be a danger of
selecting for resistance in pest insects.
In using pesticide mixture, knowledge of
cross-resistance spectrum, resistance mechanisms, genetic basis of resistance were
considered fundamental to pest resistance management strategy.
In this virtue EPA (US
Environmental Protection Agency) has recently issued the Guidance for Establishing A Common Mechanism of
Toxicity for Use in Combined Risk Assessments (February 11, 1997)[6].
Although this guidance has not yet addressed the selection of endpoints for
risk assessment and quantification, it has included mechanism
of toxicity (which is defined as the major steps leading to an adverse health
effect following interaction of a pesticide with biological targets), common
toxic endpoint, cumulative effect, structure-activity relationships (SAR)
analysis for the study of correlation between chemical structure and biological
activity.
The followings are the recent reports and
findings on the implications of insecticide mixture applications.
Mixtures May Delay the Resistance Development[7]
Synthetic
pyrethroid, fenvalerate and the organophosphate sumithion mixture may delay
resistance development in the
peach-potato aphid as reported by Wu Jinquan of the Chinese Academy of
Agricultural Sciences, Beijing. Colonies of peach-potato aphids (Myzus persica) was treated with
fenvalerate, sumithion and mixture of both insecticides (3 : 7. Resistance
level of each colony was bioassayed with the topical application procedure
recommended by FAO. After 14 generations of selection the level of resistance
of the aphids to fenvalerate developed 52.6- fold resistance and to sumithion
11.1-fold while the mixture developed only a 3.5-fold resistance.
Mixtures delay resistance development and increase
efficacy[8]
Jiang
Yanchao and Liu Runxi of the Qindao Horticulture Research Institute, China
reported on the effect of pesticide mixtures versus single applications on
resistance in the cotton bollworm. In the major producing areas of China, farmers
have adopted pesticide mixtures as the main means to control resistant cotton
bollworm , Helicoverpa armigera
(Hubner). Colonies of resistant bollworm were exposed to one of two insecticide
groups: cyhalothrin, phoxrin and parathion-methyl or cyfluthrin, endosulfan and
quinalphos, either to individual
insecticide alone; mixtures of two insecticides, or all three insecticides, for
20 generations. In the insecticide
group no 1 (cyhalothrin, phoxrin and parathion-methyl) resistance increased
very slowly when Helicoverpa was
exposed to a mixture of all three insecticides. In contrast, resistance
increased rapidly when Helicoverpa was exposed to selection with only one
insecticide. Meanwhile, intermediate increases in resistance occurred in each
colony exposed to a two-insecticide mixture.
Efficacy
of the three insecticide mixture changed slightly after 20 generations of
selection. In contrast, with single insecticides, the control efficacies for
those insecticides dropped to only 6-15%. Meanwhile selection with
two-insecticide mixtures lead to a reduction of control efficacy of between
41-65%. In the insecticide group no 2 (cyfluthrin, endosulfan and quinalphos),
the same general trends were observed. However, resistance development was
slower and loss in control efficacy was less than in group no 1 insecticides.
In part, this may be explained by the fact that in China these pesticides were
introduced later. In both insecticide treatment groups, resistance developed
slower and the control efficacy maintained longer in a three-insecticide
mixture compared to a two-insecticide mixture. Resistance developed most
rapidly when Helicoverpa was selected
with a single insecticide. These results support the decision of farmers in
China to switch to insecticide mixtures as the main method to control resistant
cotton bollworm populations.
Resistance Management in Cotton Bollworm (Lepidoptera: Noctuidae)[9]
Jinliang
Shen et al. of the China National
Center for Monitoring Pesticide Resistance, and Department of Plant Protection
Nanjing Agricultural University reported the results of monitoring pyrethroids resistance in
cotton bollworm, Heliothis armigera
Hubner, to organochlorines and organophosphates. Their studies on selection for
fenvalerate resistant and susceptible strains, consisted of the aspects of
cross resistance spectrum, the resistance mechanism and the genetics of
fenvalerate resistance. The results indicated the level of resistance to
fenvalerate in Heliothis armigera was the highest and that three possible mechanisms contributing to
fenvalerate resistance i.e., MFO factor (the major mechanism), decreased
penetration and nerve insensitivity. The resistance in cotton bollworm appeared
to be controlled by three autosomal genes.
Since the major genes involved were incompletely dominant which suggest
that the MFO gene primarily responsible for fenvalerate resistance is
incompletely dominant. Based on these findings they suggest the following IPM
strategy for the control of cotton bollworm:
(a)
The
strategy must be based on restricting the use of pyrethroids and pyrethroid
mixtures. Pyrethroids should be used only on the first-second instar larvae and
in only one generation of the season. In North China, the use of pyrethroids,
such as fenvalerate, against cotton bollworm must be stopped in those areas
with high levels of resistance.
(b)
Rotation
of chemical groups is an important tactic, especially those chemicals that show
no cross resistance.
(c)
Mixtures.
In North China, especially in the areas of high level of resistance to
pyrethroids, cotton farmers like to use insecticide mixtures for controlling
cotton bollworm, because mixtures are more effective than using one insecticide
alone.
(d)
Non-chemical
tactics of IPM for the control of Heliothis
armigera should be enforced, such as attracting and killing adult moths
with pheromone and high-voltage mercury lamps, cultivation and irrigation in
winter to destroy diapausing pupae, removing top foliage of cotton plant with eggs
of cotton bollworm in the cotton late growing season.
(e)
Proper mixture may be effective in controlling
mite infestations[10]
Bynum,
Archer and Plapp (1997) evaluated the responses by Banks grass mite, Oligonychus pratensis (Banks), and
twospotted spider mite, Tetranychus
urticae Koch, to insecticides alone and in different combinations. The
twospotted spider mite was more tolerant to many insecticides than the Banks
grass mite. Amitraz and S,S,S-tributyl phosphorotrithioate (DEF) synergized
organophosphorous insecticides diazinon and dimethoate, and the pyrethroid
bifenthrin. Piperonyl butoxide (PB) antagonized the acaricidal activity of
esfenvalerate against the Banks grass mite. In contrast to twospotted spider
mites, amitraz did not synergize the organophosphorous insecticides, and
piperonyl butoxide was a good synergist of the pyrethroid esfenvalerate
(98.3-fold increase). DEF synergized bifenthrin and dimethoate against the
twospotted spider mite. Mixtures of organophosphorous + pyrethroid mixtures were
generally better for twospotted spider mites than Banks grass mites.
Insecticide toxicity of the organophosphorous + pyrethroid mixtures was
increased from 2.3 to 18.2 times. The synergistic and antagonistic activity of
the insecticide + synergist mixtures and organophosphorous + pyrethroid
mixtures resulted in twospotted spider mites being less tolerant to several
mixtures than were the Banks grass mites. A three-chemical mixture of
dimethoate and bifenthrin with amitraz or piperonyl butoxide caused 52.7- and
94.7-fold increases in toxicity against the twospotted spider mite. The
synergistic activity by the chemical mixtures suggests that metabolic
degradation and target site insensitivity may be involved in twospotted spider
mite resistance. The proper insecticide mixture may be an effective tool for
managing Banks grass mite and twospotted spider mite infestations.
Multiple resistance to classes of insecticides and
resistance to pyrethroids and suggests use of insecticide mixture[11]
G.W.
Elzen et al., USDA-ARS Southern Insect Management Laboratory, Stoneville,
Mississippi, etc.) evaluated the resistance within classes of insecticides in
tobacco budworm, Heliothis virescens
collected in Mississippi. The temporal development of resistance showed various
patterns of cross-resistance among different classes of insecticides. High
levels of resistance were found to cypermethrin and endosulfan. Significant
levels of resistance were also shown to carbamate and organophosphorus
insecticides, although levels were generally not as high. Tests using piperonyl
butoxide did not show synergism of cypermethrin in some cases, nor was
synergism seen with thiodicarb or profenofos. Multiple resistance and the
resistance to pyrethroids appears to be increasing.
Martin
et al. (LSU Agricultural Center,
Louisiana Agricultural Experiment Station, Baton Rouge, LA) reported on the
evaluation of insecticide resistance and the effect of selected synergists in
tobacco budworm. The insecticide used were carbamate, cyclodiene, organophosphate
and pyrethroid, and the synergist piperonyl butoxide. Result suggest that
metabolic resistance to pyrethroids is widespread in tobacco budworm
populations.
A.M.
Younis et al. LA Agricultural
Experiment Station, LSU Agricultural Center, Baton Rouge LA studied the
biochemical and physiological mechanisms of pyrethroid resistance in Heliothis virescens found that multiple
mechanisms (biochemical and physiological) are associated with pyrethroid
resistance and that levels of expression of these mechanisms in the pest
populations may fluctuate during the growing season.
Effects
of single generation selection with insecticides on resistance levels in
Louisiana Tobacco Budworm were studied by B.R. Leonard et al. (LSU Agricultural
Center, Louisiana Agricultural Experiment Station, Baton Rouge, LA). Tests on
tobacco budworm resistance to pyrethroid (cypermethrin), organophosphate
(profenofos), or the carbamates (thiodicarb/methomyl) showed the budworm colony
possesses significant levels of resistance to the insecticides, and suggest
that selection with one class of insecticides can influence tobacco budworm
susceptibility to insecticides representing different classes in subsequent
generations. However, the results do not indicate that a different insecticide
use strategy would be more successful. Alternation among all classes of
insecticides recommended for control of tobacco budworm on cotton with a
greater emphasis on insecticide mixtures.
Pesticide mixtures may increase chances for toxic reactions to people[12]
Studies of certain chemicals
used by soldiers in the Gulf War of 1990-91 show that drugs and pesticides may
work in concert to defeat the body's defense systems and cause devastating
health effects, the Associated Press reported. While the most recent study, to
be published in next month's Journal of
Toxicology and Environmental Health, is most relevant to the thousands
of soldiers suffering from Gulf War Syndrome, it also throws up a caution flag
to consumers. One of the chemicals is the widely used insect repellent, DEET
and the common pesticide permethrin work by similar means.
Mohamed Abou-Donia of Duke
University found that chickens exposed to various combinations of DEET, permethrin, and the drug pyridostigmine had
diarrhea, shortness of breath, stumbling and other symptoms. Chickens have
nervous systems similar to humans, and are the preferred animal to model
nervous system effects, and autopsies showed the chickens had inflamed or
permanently damaged nervous system cells. Abou-Donia's tests showed the
chickens most harmed had less of a natural cleansing enzyme called plasma
butyrylcholinesterase, or BuCHE. This enzyme normally filter out chemicals like
DEET or permethrin before they can get into the brain.
Evidence of the effects of
the combined chemicals was first found in a Gainesville US Department of
Agriculture laboratory by Dr. James Moss in 1993, however the USDA did not
think the findings were important and discontinued the research.
[1] Presented in the International Workshop on Insecticide Mixture Formulation, Hilton-Jakarta, December 9, 1997.
[2] Professor, Bogor Agricultural
Institute (IPB) and UKRIDA, Jakarta
[3] EPA Pesticide Regulation (PR) Notice 97-3, September 4, 1997
[4]Klaassen, C.D. 1986. Principles of Toxicology. In: Toxicology, the Basic Science of Poisson, C.D.Klaassen et al. (eds.). Mcmillan Co.
[5]Metcalf, R.L. 1982. Insecticides in Pest Management. In: Introduction to Pest Management, R.L. Metcalf and W.H.Luckman (eds.). John Wiley & Sons.
[7]
Resistant Pest Management, Vol. 7, No. 2;
Winter 1995, A Biannual Newsletter of the Pesticide Research Center (PRC) in
Cooperation with the Insecticide Resistance Action Committee (IRAC), http://www.msstate.edu/Entomology/v7n2/art05.html.
[8] Resistant Pest Management, Vol. 7, No. 2; Winter
1995,
[9] Resistant Pest Management, Vol. 6, No. 1,
Spring 1994,
[10]
E. D. Bynum, Jr., T. L. Archer and F.W. Plapp,Jr (1997): Comparison of Banks Grass
Mite and Twospotted Spider Mite (Acari: Tetranychidae): Responses to
Insecticides Alone and in Synergistic Combinations. Accepted Manuscripts for Journal of Economic Entomology, Posted
on internet 6/6/97, at http://www.entsoc.org/jeeram.htm
[11]
FAO/IAEA Consultants Group Meeting.
Resistant Pest Management, Vol. 6, No. 1, Spring 1994, A Biannual Newsletter of the Pesticide Research
Center (PRC) in Cooperation with the Insecticide Resistance Action Committee
(IRAC), http://www.msstate.edu/Entomology/v6n1/spring94.html/art19
[12] Today's Florida Environmental News, Wednesday, April 17, 1996, Posted 10:30 am, 4-17-96, at http://www.enviroworld.com/Apr96/041796.html#anchor701056