How do anticholinesterase pesticides kill nematodes?

Question

Compounds that inhibit the enzyme acetylcholinesterase are commonly used as pesticides. In animals with centralized respiratory systems controlled by the nervous system, poisoning with an anticholinesterase would lead to death due to respiratory arrest. I presume that, in insects, the inhibition of AChE would lead to closing of the spiracles necessary for respiration

However, nematodes and other worms rely on diffusion of gasses for respiration and do not have respiratory systems controlled by a nervous system, so how would inhibition of AChE by agents such as aldicarb kill them, especially if the pesticide in question happens to be a reversible inhibitor?

Answer 1

Cholinesterase inhibitors and plant-pathogenic nematodes

While it is true that cholinesterase inhibition does not affect gas exchange in nematodes, it does produce other effects by paralysing motor activity:

• Inhibition of larval hatching

• Reduced movement to new roots

• Impairment of root invasion and feeding

• Impairment of copulation

Acetylcholine also has a role in sensory neurotransmission in nematodes, which could further contribute to the toxic effects.

Thus cholinesterase inhibitors do not directly kill nematodes, but only slow down their growth and reproduction. This is why some prefer to use the term 'nematostatic' over 'nematicidal' for these compounds.

Effects on vertebrate pathogens

Pyrantel is a cholinesterase inhibitor used to treat intestinal nematode infections in man and other mammals. Little is known about how exactly it works, except that it causes spastic paralysis of the worms. Possibly, it impairs feeding and reproduction (like its counterparts in plant pathogenic nematodes).

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References and further reading:

1. Wright DJ. Nematicides: modes of action and new approaches to chemical control. In Zuckerman BM, Rohde RA, editors. Plant parasitic nematodes. Vol. 3. New York: Academic Press; 1981. p. 421-449. Link to publisher site.
2. Martin RJ, Robertson AP. Mode of action of levamisole and pyrantel, anthelmintic resistance, E153 and Q57. Parasitology. 2007 Aug 1;134(8):1093–1104. https://doi.org/10.1017/S0031182007000029

Answer 2

In nematodes, Acetylcholine is the main excitatory neurotransmitter at neuromuscular junctions. Large amount of nervous system cells in Caenorhabditis elegans secrete acetylcholine for Cholinergic neuromuscular transmission.

Cholinergic transmission is involved in various physiological processes in many C. elegans, such as locomotion, egg laying, feeding, and male mating.

So acetylcholinesterase which is an enzyme responsible for hydrolysis and inactivation of acetylcholine plays an essential role in regulation of cholinergic transmission. Thus inhibitors of this enzyme can act as nematocide.

Reference:

http://www.wormbook.org/chapters/www_acetylcholine/acetylcholine.html#:~:text=Acetylcholine%20is%20the%20major%20excitatory,elegans%20nervous%20system%20release%20acetylcholine.

https://pubmed.ncbi.nlm.nih.gov/16569291/#:~:text=Acetylcholine%20is%20the%20major%20excitatory,for%20regulation%20of%20cholinergic%20transmission.