Sunday, July 24, 2011

Beware the Resistance

I was reading an article the other day from the University of Florida about Citrus Greening and the insect that spreads it, the Asian Citrus Psyllid. The article was more about the worry that these critters are starting to show resistance to the insecticides. The levels that have been documented to date are not high enough to cause product failures. Yet.
This got me to thinking about the insecticides that the average homeowner is using. In Florida and anywhere else commercial crops are grown, the farmers have a vast array of insecticides at their disposal. The homeowner, not so much.
Let me start with a couple of definitions.
Insecticide: a substance used for killing insects.
Insecticide resistance: an increase over time in the ability of an insect population to survive an insecticide application.
Some insects are going to naturally have a resistant gene to certain chemicals. Think of the situation that some people are allergic to peanuts and others are not. The ones that are not, have a resistance to the peanut chemical. This is a weak example, but gets my point across.
Insects with a resistance gene survive an application of insecticide and pass the resistance trait on to their offspring. If you continue using the same kind of insecticide that has the same Mode of Action,(more on that in a minute)the resistant individuals will continue to breed and the proportion of resistant insects in the population increases, while susceptible, or ones that don't have the gene are eliminated by the insecticide.
I know, what the heck did he just say?!
Pesticides are seldom 100% effective, so there are always a few individuals that can survive and reproduce. Survivors may have been able to detoxify the pesticide or are immune to the effect of the pesticide or can avoid the pesticide application altogether. If survivors mate and past on this resistance to their offspring, future generations will have fewer susceptible individuals. As time progresses, the entire population may become resistant.
This brings me back to the MoA or Mode of Action. Insecticides all work in different ways. I am not going to get major geek on you here, but some of the ways that they work and on what part of the insects body are,
The Nervous System: Most traditional insecticides attack the nervous system of insects. It interferes with their nerve fibers or the gap between the fibers.
Energy Production: These chemicals significantly slow the production of energy that an insect needs to survive. It basically starves them. The insects “run out of gas” and can even die standing on their feet.
Cuticle Production: Insects wear their skeleton on the outside of their body. Chitin is a major component of their skeleton. Chemicals called chitin synthesis inhibitors,slow or stop the production of chitin. Without chitin, the insect cannot molt,(shed it’s exoskeleton and re-grow a new one) and will soon die.
Endocrine System: Insect growth regulators,act on the hormone or endocrine system of insects. Juvenile hormones keep insects from molting (shedding their exoskeleton) until the insect reaches the proper state of maturity. They don’t allow the insect to molt at the proper time, keeping the insect from maturing and reproducing.
Water Balance: Insects have a thin waxy coating over their exoskeleton to prevent water loss. Some chemicals absorb the waxy coating, resulting in rapid water loss and eventual death.
There are many more, but you get the idea.
So now, you are wondering, how does this affect me? The chemicals are broken down into groups, 25 or so to be exact. You need to alternate between groups, NOT just name brands.
Here is an example of what I mean. Group one is Carbamates and Organophosphates, it is broken down into sub-groups A&B. Stay with me here. Sevin, a common insecticide, is in this category. A good product. It interrupts the transmission of nerve impulses. So, to avoid a resistance problem, you want to switch to something else....Good Idea!
You grab some Malathion...Bad Idea! This is in the same group and is the same way of killing the bugs. Ideally, an effective insecticide should be applied at a concentration high enough (following the label directions) to kill all the individuals in a population. However, it should only be applied twice before switching to another group.
Maybe going to the Insecticidal soaps, which will dry the insect out. You can also go to the Horticultural oils, which will suffocate the insect. Okay, so now you are asking, how do they become resistant to the soaps and oils? They will develop a sense of the danger and will move away from the chemical. If you do not hit them directly they may move to the under side of the leaf or a part of the plant that was not sprayed.
I know this may sound a little confusing, and yes it probably is WAY too detailed.
My point today is this, try to attract beneficial bugs to your yard first. Flowers and plants that they like are a good step forward. If that doesn't help, start with the most gentle of insecticides, soaps and oils. If you must go to the nuclear option of chemicals, keep in mind that you might be contributing to the evolution of a Super Bug. Then a nuclear bomb might be the only way to rid yourself of them!
Your best bet is to get to know a little bit more about the different classes and how the Modes of Action work. A good website is: MOA's
Oh, and just as a side note....Fungicides can also have resistance issues, but that is fodder for another blog.
Happy Growing!

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