There are many similarities in
fungicide resistance, insecticide resistance, and
herbicide resistance. The terminology is a little bit
different among plant pathology, entomology, and weed science, but
the concepts are very similar. In this presentation, we use the
terms insecticide resistance and pesticide resistance
interchangeably. In entomology, we deal with insecticides and
acaricides (or miticides), but sometimes it’s easier just to
say “pesticide” resistance when referring to these even though
pesticides refer to a broader range of materials. What is pesticide resistance?
It is a decreased susceptibility of a pest population to a
pesticide that was previously effective at controlling the
pest – the pesticide is no longer working as well. A more
technical definition of insecticide resistance is a
heritable change in the sensitivity of a pest population
that is reflected in the repeated failure of a product to achieve
the expected level of control when used according to the
label recommendation for that pest species. What is a heritable change? It
is a change in the genes of individuals in the present
generation that is passed on to the next generation. Resistance develops in
individuals of a population. A population is a group of
individuals of the same species living at a given time and at a
given place. For example, two- spotted spider mites on corn
leaves in one part of Tulare County is a pest population.
That population may be different from two-spotted spider mites on
corn leaves living in another part of Tulare County. Different
populations can have different traits and characteristics such
as different resistance levels to various toxicants. Resistance
happens at the population level. Insecticide resistance is not a
new occurrence. DDT started the era of synthetic insecticides
when it was discovered in 1939. Paul Muller discovered the
properties of DDT and won the “Nobel Prize in Physiology or
Medicine 1948.” In 1947, house flies developed resistance to
DDT and later many other insects and mites became resistant to
this pesticide. So resistance in insects is certainly not
anything new, and furthermore, it is not something that only
happens to synthetic compounds. In 1914, scale insects developed
resistance to lime sulfur, an inorganic compound. How does resistance develop?
Let’s say you have a population of spotted wing drosophila. They
all look the same out in the field, but the boxes around the
three individuals indicate that there is something unique about
them. You can’t see it, but they may differ metabolically,
behaviorally, or structurally. They are all the same species, but
these three differ in some way. You spray these spotted wing
drosophila and what happens? You’ve killed most of the normal
ones, but it didn’t touch the three that were different in
some way. So what is going to happen to the next generation? Those three different individuals
reproduce, and you see that you have 11 of them now. You also
had one normal one, and that one produces another normal one. So
your population in this example has shifted primarily from the
“normal” individuals to these different sorts of individuals. What happens if you spray again? You are left with the unique
ones. It killed some of the unique ones, but you don’t have
any of the normal ones left. If you go through another
generation and spray again, nothing happens. The population has switched from
primarily normal individuals to 100% of the ones with some
unique characteristic. And that unique characteristic is
resistance. The insecticide was the selection pressure and it
selected for these different individuals, but the bottom line
is the population you started out with had genetic flexibility
that created different individuals. You didn’t create
the genes, they were already there, but the insecticide
selected for the genes in these different individuals that gave
them resistance and now your population consists of 100%
of the unique ones. Genetics and selection pressure
are the two most important factors that influence
insecticide resistance. There is genetic variability in the
individuals of the insect population. Then selection
pressure, intensive application of insecticides, brings those
genetic differences to the forefront by eliminating the
susceptible individuals and causes the unique individuals
to become the majority of the population. What affects how fast insecticide
resistance develops in insects? There are a number of different
factors. The types of resistance genes, how many are involved,
and are those alleles dominant or recessive? If they are
recessive, and they mate with susceptible individuals then
resistance doesn’t manifest itself in the next generation.
If the genes are dominant, then even when the resistant
individuals mate with susceptible ones, their children
can resist the insecticide. How many generations does a pest
have per year? Pests that have many generations tend to develop
resistance faster than those with a single generation per
year. Citrus thrips with 8 generations per year developed
resistance to OPs in the 1980s, California red scale (with 4
generations per year) developed resistance in the 1990s, and
citricola scale (with 1 generation per year) developed
resistance in the 2000s. How mobile is the pest? Mobile
pests that are highly motile and mate with susceptible individuals
are less likely to develop resistance than pests that are
immobile. Once a population of California red scale developed
resistance to OPs, it tended not to lose it because there is very
little influx of susceptible individuals into an orchard –
the males don’t fly very far. The worst-case scenario is many
greenhouse insects and mites. There are few susceptible
individuals coming into the house to breed with, they have
rapid life cycles, often dominant resistance, pesticide
applications are applied frequently in greenhouses
because there is a low tolerance for damage. Resistance can
develop very fast in greenhouses. The persistence of the pesticide
residues and the overall selection pressure imposed is
another important factor. The more you spray and the more
persistent the pesticide, the quicker the selection
of resistance. Insecticide resistance is
different from insecticide tolerance. Tolerance is the
natural ability of a population to withstand the toxic effect of
an insecticide. Not all insecticides kill all insects
from all species. For example, if you use imidacloprid, and try
to kill beet armyworm, it isn’t going to kill these larvae very
well. Never did, never has, never will. Beet armyworm as a species
is naturally insensitive or tolerant to it. The biocontrol
agent green lacewing can walk through pyrethroids and is
unaffected because it naturally has very high esterase enyzmes. How common is resistance
worldwide? There are about 600 species of insects and mites
that are known to have developed some level of pesticide
resistance. There are literally dozens of insecticides and
miticides to which two-spotted spider mites, diamondback moth,
Colorado potato beetle, and sweetpotato whitefly have
developed resistance worldwide. Colorado potato beetle is found
in most other states, except California, and this graph shows
the number of different active ingredients that Colorado potato
beetle has developed resistance to. You can see the steady
progression over the years. There is no stable insecticide
management for this insect. If you have an insecticide that
works for one year, chances are that it won’t work the next year
or at least the year after that. This insect just has the perfect
set of characteristics that allows the insecticide
to go right through. Insecticide resistance develops
in agricultural pests, medical and veterinary pests, and also in
natural enemies. We don’t mind when natural enemies such as
predators and parasites develop resistance because it allows
them to survive the pesticide and help control the pests. But
resistance in natural enemies doesn’t happen as often
as the pests. What taxa are involved in
resistance development? Many species of Diptera (flies and
mosquitoes), Lepidoptera (caterpillars), Acari (mites),
Coleoptera (beetles), and Hemiptera (sucking insects) have
developed resistance to pesticides. Why is this so common in insects?
Insects have been, and still are, extremely adaptable organisms.
There are about 900,000 different species of insects in the world
representing about 80% of the world’s species. They are
survivors, they’re adaptable, and they evolve. Insecticides are organized into
classes, such as organophosphates, carbamates, pyrethroids,
neonicotinoids, and others. Within each of these classes,
they share a similar chemical structure and a common mode of
action. That is, all the insecticides within a group have
a similar way of killing the insect. There are currently 28 different
groupings of insecticides or miticides – most of them attack
the nerves and/or muscle system in the insect. That has been one
of the most fruitful ways to kill insects – design insecticides to
attack the nervous system. But insecticides have also been
designed to attack their molting process, respiration, and midgut.
And with some insecticides, we don’t know how they work. The Insecticide Resistance
Action Committee (IRAC) categorizes pesticides into
these classes. Chemical companies are always trying to
develop insecticides that fall into new groups and work by a
different mode of action to get around some of the
resistance issues. Group 3 insecticides are the
pyrethroids. All of these have the same Mode of Action – that
is they attack the sodium channel in the insect nervous
system. Because they attack the same part of the insect, if an
insect population develops resistance to one of the
materials in the group, it will be resistant to all
of the pyrethroids. The mode of action is the
process by which an insecticide kills an insect, or inhibits its
growth. The target site is the exact location where the
insecticide works. If the mode of action affects the nervous
system, the target site might interfere with one certain
enzyme within the nervous system. The mode of action and target
site are often used interchangeably, but they do
differ a bit in what they mean. As said previously, there are 28
different modes of action. Eleven of them work at the
nervous system or muscle system. Seven of them affect growth and
development of the insect. These are the insect growth regulators
that prevent egg hatch, or molting by larvae and pupae or
prevent the development of the chitin exoskeleton. The muscle system mode of action
is the latest so-called “hot” area of development. Ryanodine
receptor modulators is a fairly new mode of action on the market
for the last several years. Coragen® and Verimark™ are two
examples. They are called diamides. There are 6 different classes of
insecticides that work at the insect respiration, or
energy, systems. Then there are some that work in
the midgut of the insect. Those are primarily the
Bacillus, or Bt, products. There are several ways that
insects (and mites) develop resistance to insecticides. One
is called metabolic resistance. Resistant insects can detoxify
or break down the toxin by increasing the number or type of
enzymes they have in their bodies. These enzymes then bind to the
toxin and prevent it from poisoning the insect. This is
the most common type of resistance in insects and it
presents the greatest challenge for resistance management. Another type of resistance is
behavioral resistance. One portion of the population
behaves differently than another portion and is selected out by
the pesticide. Mosquitoes are adept at this. Some populations
have developed a preference to rest outdoors because the
portion that naturally rested indoors was eliminated. House
flies can be selected to do the very same thing. Altered target-site resistance
is another general way that insects can develop resistance.
The site where the toxin usually binds in the insect becomes
modified to reduce the insecticide’s effect. Finally, penetration resistance
is another one. Insects with a thicker cuticle survive because
it slows the insecticide from penetrating the body
of that insect. Now let’s go through a couple of
different modes of action to show how resistance has
developed. Organophosphates and carbamates affect the insect
nervous system. The diagram shows the insect nervous system.
You can see the brain. They have a small brain, central nerve
cord and small axons coming off to regulate the movement of the
wings and legs. The one difference between the insect
nervous system and the human nervous system is that the insect
one exists in several units. The gap between nerve axons is
called a synapse. In the insect nervous system, a signal comes
down the axon, then it has to jump the synapse, and the signal
goes to the next axon, and so on. So the insect nerve is not just
one cord or complete line. It’s a series of subunits. When a
signal reaches the synapse, the compound acetylcholine is
released. This is a neurotransmitter, a messenger,
or signal. That neurotransmitter jumps the synapse and plugs into
the receptors on the next side and the signal goes on. The
signal may be to move the leg or wing. After the signal has been
transmitted to the other side, the enzyme called
acetylcholinesterase, resets the synapse and turns the signal off. If an OP or carbamate insecticide
is in the insect system, it binds with the acetylcholinesterase so
the synapse can’t reset and the signal keeps on transmitting
across the synapse. The nerve stays in the excited transmitted
state and the insect can’t function normally. When the
insect is exposed to the insecticide, the nerves continue
to fire and the insect has tremors, convulsions, and
paralysis. In resistant individuals, they have increased
levels of esterases, some for the pesticide and some for normal
function of the synapse, so that it stops firing and the insect
returns to a normal state. Another group of insecticides is
the neonicotinoids. These also affect the nervous system but in
a different way. Acetylcholine is the chemical that crosses the
synapse and causes the signal to be transmitted. Neonicotinoids
plug into these receptor sites and block the acetylcholine. The
acetylcholinesterase cannot reset the synapse because it can’t
break the bond between the neonicotinoid and the receptor
and the signal continues to be transmitted, i.e., the insect
continues to move its legs, for instance. Resistance in neonicotinoids has
been very slow to develop. Scientists have found an enzyme
detoxification mechanism and some target site protein changes
in insects that are known to be resistant to the neonicotinoids.
This suggests that multiple mechanisms and multiple genes
are involved, but resistance has been slow to occur, suggesting
the genetics are not dominant. Insect growth regulators are
compounds that occur naturally in insects (and not humans).
Scientists were able to mimic those, reproduce synthetic ones
in the laboratory, and take advantage of weak points in the
developmental process of the insect. There are 3 or 4 classes
of IGRs. There are chitin synthesis inhibitors that disrupt
the skeleton production of the insect, and there are hormonal
IGRs such as juvenile hormone mimics that keep the insect
juvenile, never reaching adulthood and molting hormone
agonists that prevent the insect from growing through molting. The insect body contains a
substance called chitin. We have our skeleton inside our body,
but insects have their skeleton on the outside of their body,
basically a suit of armor. That skeleton in the insect is
composed largely of chitin, which is a very tough, durable,
type of product. The only way that an insect larva (or nymph)
can get larger is to shed that skeleton. When it does that, we
call it molting. Each time an insect molts, the exoskeleton
gets larger and the new exoskeleton hardens up and
contains chitin. So if the insect has been exposed to a
chitin-synthesis inhibitor, it molts but when it tries to form
its new exoskeleton, it can’t and the insect contents ooze out.
There is also chitin in insect eggs, so there is some activity
killing insect eggs as well. Juvenile hormone mimic is one of
the hormonal types of IGRs. Juvenile hormone is a compound
that when it is at high levels in an insect larva and the larva
molts, it progresses to another larval stage. When it wants to
molt from a larva to a pupa, JH must be at a low level in the
insect. This is how the insect is “designed.” If you’ve exposed
the insect to artificial Juvenile Hormone when everything else in
the insect is telling it to go to the pupal stage, the insect gets
crossed signals and it could stay a larva or only partially
develop into a pupa and die. Resistance to insect growth
regulators has been fairly slow to develop since these are
compounds that naturally occur in these insects. California red
scale has been treated with the insect growth regulator
pyriproxyfen on a yearly basis since 1998 in many orchards, but
resistance has been difficult to document in the laboratory. The newest group of insecticides
to come on the market, group 28, are the Diamides. They bind to
ryanodine receptors in the muscles, activating them to
release stored calcium. The calcium is needed for muscle
contraction and without it the insect becomes paralyzed. There has been resistance to
ryanodine insecticides found in diamondback moth (in several
Asian countries). The resistance appears to be due to a change
in the target site. So how do you know if the insects
have resistance or not? In the past, you would have used a
hand-held microapplicator, filled it with insecticide, and
tested laboratory-reared insects with different doses. This was
very laborious. You needed a large number of insects. Another way it is commonly done
is looking at field trial efficacy. If you look at these
data on blue alfalfa aphids from the Imperial Valley, you see the
number of aphids in the check plots verses 5 commonly used
insecticides. The insecticides are not killing all the insects,
but it is hard to say if it is due to resistance. Another example with blue
alfalfa aphid. Looking at the field data, you might say there
is some resistance with Lorsban on the March 28 date but with
all other factors involved, it is hard to interpret field data. A third way to determine if
resistance is occurring is with laboratory bioassays. There are
currently hundreds of bioassay methods. It depends on insect
species or what system you’re working in. It involves dipping
leaves, dipping fruit, or coating inert substrate such as
petri dishes with different doses of insecticides. Other examples
are coating bottles or plastic bags. Diet incorporation is
another type of bioassay. In these bioassays, you place
the insects on the pesticide- treated surface and determine if
they live or die after an interval of time such as hours,
days, or weeks. These are relatively inexpensive, the
results depend on the method used, and you need
a source of insects. Biochemical techniques can be
used if the resistance has been documented to be due to an
increase or change in the enzyme. Molecular or DNA assays or ELISA
can measure an enzyme or other compound in the insect to
determine if it is resistant. California red scales that are
resistant to OPs, have higher esterase enyzmes. The level of
esterase enzymes can be measured by grinding up individual scales
and mixing them with chemicals that turn dark in the presence
of these enzymes and the proportion of resistant
individuals in a population determined. What do you do when you collect
these data? Scientists measure the resistance in a population.
They look at the response of the insect to a series of
concentrations of pesticides and calculate the LD50, which is the
concentration of pesticide that kills 50% of the population. The
resistance ratio is found by dividing the LD50 of the
resistant population by the LD50 of the susceptible population. In this example, it takes about
2 ppm chlorpyrifos to kill a susceptible California red scale,
but it takes about 70 times that amount to kill a resistant
population of scales, so the resistance ratio is 70x. Here’s an example with the rice
water weevil. There have been reports by PCAs that the compound
Lambda-cyhalothrin was no longer working. Weevils tested from 2
different sites had LD50 values of 0.096 and 0.32 ppm. This
suggested that there was a 3 to 4 fold resistance between the two
sites. But you can’t tell if there is resistance occurring
unless you have something to compare it with before the
chemical was used heavily. Fortunately there was some data
from 1999 when this product was first put on the market that
suggested that it took 1.5 ppm to kill this insect. Insects
were more susceptible to the compound than back in 1999! This
shows that when these compounds first come on the market,
you really need to develop baseline data. A discriminating dose or
concentration is another way you can develop resistance data. You
test a series of different doses and develop a line that shows
when you kill more than 99% of individuals and use that dosage
to discriminate between susceptible and resistant
individuals. If they survive it, then they must be resistant. So how do you manage resistance
for insecticides? The goal is to reduce selection pressure. How
do you do that? Avoid applying insecticides is of course the
best way but in most cases is not practical. Use non-chemical management
tactics as much as possible, such as preserving or augmenting
beneficial organisms. Minimizing the number of applications is
another way. Rotate insecticide modes of action – change it up
on the insect. So if you’ve used a group 18 product, next time
use group 20. Unfortunately, the number of products and the
number of modes of action are limited for most pests. Using a
high label rate tends to select resistance slower than a low
label rate because it does not allow many of the insects
to survive. If you look at insecticide
labels, they tell you what group the insecticide falls into. So
you know right away what mode of action the group falls in. Use good IPM practices.
Monitor pests. Use research-based sampling
procedures to determine if pesticides are necessary (based
on action or economic thresholds) Use the best application timing
(when pests are the most susceptible) Employ appropriate control
measures. Effective IPM – based programs will include
insecticides, cultural practices, biological control
(predators and parasites), mechanical control and sanitation. Select and use
insecticides wisely. If repeated applications of
pesticides are necessary, alternate insecticides with
different modes of action against the pest so that no more
than two consecutive applications are made with the same mode of
action. The insecticides used in a rotation or tank mix must be
active against the target pest. Follow label directions for the
proper application method and rate. There are trade-offs in
using low rates to preserve natural enemies and increasing
the selection pressure for resistance. Minimize the use of
long-residual insecticides because that extends the period
of selection pressure. When persistent pesticides must be
used, consider where they can be used in a rotation scheme to
provide the control needed and with a minimum length of
exposure. Select insecticides that are least damaging to
populations of natural enemies. When feasible, spot treat or
leave unsprayed areas within treated fields or adjacent
“refuge” fields. The pesticide – susceptible individuals in the
untreated areas will interbreed with resistant ones and dilute
the resistance genes in the population. Keep good records of
insecticide use to aid in planning for future years. Tank mixes or premixes are
becoming more and more common in entomology and other disciplines.
From the entomology stand point, premixes aren’t a good thing.
A PCA or grower may take insecticide A from group 3 and
insecticide B from group 10 and using the appropriate rates, do
a mix. But if you are buying a premix in a jug, many times the
rates are not optimal for both products, and one of the
pesticides in the mix may not be targeted for the pest you
intend to control. It is better to purchase the
insecticides as individuals and mix them for the target pests
you are interested in controlling. In addition, it is better to
apply two pesticides days or weeks apart in a rotation. With
two separate applications, you cover more of the host plant and
extend the effect of the insecticide compared to when you
apply the two chemicals mixed together in one application.
In weed science, everything is tank mixes. You control a broad
spectrum of weeds. You mix herbicides that have a different
mode of action but similar activity on the same species.
You eliminate the possibility to develop resistance on each of
those but can eventually have the possibility of
multiple resistance. There are four scenarios of
rotation of pesticide seen in this slide. Scenario 1: If there
is no rotation of insecticides, selection pressure will
be very fast. Scenario 2: If there is rotation
within generations, the insect is being ‘stressed’ by two
different chemicals in the same generation and resistance
is slowed. Better yet, Scenario three is
rotating between generations to allow recovery of susceptibility
over an entire generation and should slow resistance even more
than scenario two. Scenario four is rotating both
within and between generations – this is the ideal. Often, there is a cost to the
insect to develop resistance. If they have to make more
enzymes or different enzymes to withstand the pesticide, they
may produce fewer or smaller progeny or live a shorter time.
This is important, because when the sprays (or selection pressure)
stops, often the susceptible individuals are more fit and the
population can gradually shift back to less resistant. Let’s look at some success stories.
Spider mites and abamectin in cotton. Abamectin has been used
for 20 plus years in California. Two-spotted spider mites have
developed resistance very rapidly to many materials in many areas
and crops. Abamectin still works to control mites and is still a
top-used product in California cotton. There was a blip of
resistance seen in the 1990’s in a bioassay, but it wasn’t seen
again. Why is this so successful? Abamectin used to be a very
expensive product. So that limited use to some extent. The
treatments in the California cotton system were used on a
timely manner. They were applied mid-June, not waiting until you
have a large population of mites or when the cotton is huge to
inhibit good coverage. A lot of cultural controls are also used
in cotton to control spider mites so that has kept resistance down.
Contrasting this with a situation in southern cotton in Louisiana
and Mississippi. They’ve had spider mites for 5 years now.
They started using abamectin and other products but now have
resistance to abamectin. They didn’t use full rates and only
did cheaper treatments. They didn’t monitor their fields so
only applied the product when they had a full – blown problem.
Their lack of management of the insecticide has cost them. A very similar success story is
with the neonicotinoids for melons and vegetable crops for
whiteflies in the southern California desert areas.
Whiteflies tend to develop resistance very fast. These are
soil-applied, long-lived treatments so can favor
resistance to develop very fast. After using these products for
20 years, they are still effective at keeping whiteflies
under control in this area. BT cotton, or genetically
modified cotton, for Lepidoptera is another success story. This
is not in California but in the rest of the U.S. It is
highly regulated, highly researched. They have
regulations in place to produce susceptible individuals. There
are other technologies in the system. There has been some
slight slippage in susceptibility but still highly effective with
no real resistance seen. Contrasting this with a similar
situation in India. They have the same products, but after
about 5 or 6 years, they’ve got full-blown resistance in their
Lepidoptera insects. They had the same rules as in the U.S.
but the rules were not enforced. Whitefly management in Arizona
cotton, melons, and vegetable crops. This is a highly
researched area. They do annual monitoring of resistance levels.
If they saw resistance starting to develop, they would know it.
There is cooperation among the different industries in different
commodities (public and private). There is a limited number of
effective insecticides and they are managed very well. After 20
years, the system is still in place and doing very well.
So research, cooperation, and staying on top of the situation
has helped to keep resistance down. So what does it take to maintain
susceptibility in some of these problematic situations? Good
science, good cooperation and communication within the
industries and among the PCAs. Well-trained people in the field
and good observational skills are key. Many times the growers
and the PCAs are the first people to see it and communicate
back to the researchers. Continuing to develop new management
technologies and educating the general public is important to
allow these things to be used. This concludes the presentation
on insecticide resistance. [No audio]