Answer: Both products are equally effective in killing grubs. Trichlorfon and Imidacloprid work by inhibiting certain important enzymes of the nervous system and both kill insects by contact and by ingestion. The only real difference is that Trichlorfon is an organophosphate so it is not as persistent (breaks down faster) than Imidacloprid. On the other hand, since Imidacloprid has a long residual life, it will provide longer preventative control; Trichlorfon is shorter-lived but works better against large grubs. So the jury is still out on which might be best for your customers! It depends upon their situation, whether they want some residual action, and what time of year it is (which will dictate whether the grubs are small or large). Hope I didn't confuse matters even more with this information!
If your customers are looking for a more organic solution, Milky spore disease is reportedly just as effective against grubs. Milky Spore is a naturally occurring host specific bacterium (Bacillus popillae-Dutky). This product is lethal to and specifically targets white grubs. (I don't know if your garden center carries this product so be sure to check before recommending it.)
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