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The multiple insecticide-resistant green peach aphid makes a good case for IPM

Spraying an insecticide may seem like the easiest and most effortless type of insect control, but insecticide resistance has proven that spraying without doing your homework first is not the most practical way to protect your crops. This week, a blog post from “the Aphid Room” reminded me of why the first step of pest management is identifying and knowing the pest: some insects, like the green peach aphid, are resistant to several insecticides, making control quite a challenge.

Green peach aphid adult

green peach aphid adult

The green peach aphid (Myzus persicae (Sulzer)) is a versatile pest; it exists all over the world and feeds on hundreds of plant species from over 40 families, from tree species such as peach, apricot and plum, to herbaceous hosts such as vegetable crops, flowers and other ornamental plants. In addition to damaging plants by feeding, the green peach aphid can transmit over 100 viruses. The real issue for farmers, however, is that this aphid species is resistant to over 70 different synthetic insecticides.

A group of scientists from Chile and the United States took a closer look at the mechanisms of resistance in green peach aphid (Silva, 2012) and discovered six distinct insecticide resistance mechanisms:

  1. Modified acetylcholinesterase (MACE), which allows the species to resist organophosphates and carbamates
  2. Kdr and super kdr mutations in the sodium channel, the target of pyrethroids and organochlorines
  3. Mutation of the GABA receptor, the target of organochlorines of the cyclodiene type
  4. A mutation of a key residue in the loop D region of a nAChR b1 subunit
  5. The overproduction of two esterases, which allows resistance to organophosphates, pyrethroids and carbamates
  6. The overproduction of a chytochrome that allows resistance to neonicotinoids.

The research group evaluated 32 genotypes and grouped them into 3 categories based on the type of mutation. The first category had no mutations and the lowest level of resistance of the three groups. The second had a kdr mutation only, and the third had multiple resistance mechanisms, including kdr and MACE mutations.

If, like me, you are unfamiliar with genetics, you are probably wondering what kdr and MACE mutations are and why you should even care about them. Before I define the mutations, I need to explain how insecticides affect the body of the insect. Please understand, however, that insecticides are designed to poison insects and do not necessarily have the same affect on humans (despite some of the rumors I’ve seen online).

Insecticides are divided into classes. Each class has what’s called a “mode of action,” or a specific bodily function that it targets to change in order to kill the insect. For instance, pyrethroid insecticides target the sodium channel. The sodium channel, very simply, is a door to the nerve cells that allow sodium ions to enter in certain circumstances, but close out other ions such as potassium and hydrogen ions. Sodium ions cause activity in neurons; if the sodium channel is damaged, the neurons can’t activate, or in the other extreme, the neurons become over-excited, causing seizures. Pyrethroid insecticides impair the sodium channel in the neurons.

However, the sodium channel is the source of “knockdown resistance” (kdr) mutations that make the sodium channel less susceptible to damage by pyrethroids. Insects having this particular mutation are “resistant” to pyrethroid insecticides, meaning that the insecticides are not effective for killing the insect. For a really thorough and scientific explanation of what the sodium channel is and how mutations affect insecticide resistance, go to this article on the NIH website.

Acetylcholinesterase (AChE) is another enzyme that affects neurological activity and is the target of organophosphates and some carbamate insecticides (if you’re not familiar with the insecticide classes, Radcliffe’s IPM World Textbook has a great chapter explaining them). When it is released in the body, acetylcholinesterase allows the nerves to receive signals and a second subsite terminates the signal. Organophosphates and carbamates inhibit acetylcholinesterase production, causing muscular paralysis, convulsions, and asphyxiation.

Mutations to AChE allow the enzymes to resist the inhibiting effects of organophosphates, making the pesticides less effective at killing the insects. Some Drosophila species have this mutation.

When an insect species develops mutations that render certain types of insecticides less effective, the farmer must use alternative form of pest control, such as natural predators, pesticides from other chemical classes or mechanical methods, such as mowing weeds or non-agricultural plants that may harbor the insect when the crop is not growing.

To find out how the aphids were resisting the insecticides, the researchers exposed each genetically-resistant group to pirimicarb, a selective carbamate insecticide for aphids. Category 3 (multiple-resistant) showed increased activity in genes that possessed both kdr and MACE mutations. Category 1 (sensitive) exhibited an increase in energy metabolism, possibly due to the stress response to the insecticide action and seemed to use the increased energy production to mitigate the effects of the insecticide. Category 2 showed no increase in energy production but did increase the production of detoxifying genes.

damage to leaves

aphid damage to plant

With a variety of resistance factors, the green peach aphid makes a good case for the need for an integrated management strategy. A fact sheet on the green peach aphid on the University of Florida’s website recommends a combined strategy for chemical and biological control and also offers several suggestions to alleviate damage from the viruses transmitted by the insect. Because this aphid is resistant to so many insecticides, research your control options thoroughly before making any applications.

Sources:

Mandrioli, M. “Aphid: gold medal for insecticide resistance.” The Aphid Room. Blog.

Silva AX, Jander G, Samaniego H, Ramsey JS, Figueroa CC (2012) Insecticide Resistance Mechanisms in the Green Peach Aphid Myzus persicae

(Hemiptera: Aphididae) I: A Transcriptomic Survey. PLoS ONE 7(6): e36366. doi:10.1371/journal.pone.0036366.

Capinera, J. “Featured Creatures: the green peach aphid.” University of Florida Institute of Food and Agricultural Sciences. Photos on this page are from this webpage as well.

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