The advice to use solutions of dishsoaps/ detergents is short-sighted for a number of reasons. Dawn, Palmolive, Joy, ...... dishsoap, as an example, are each and all a mishmash of chemicals, at least eleven of which are phytotoxic. If the above-ground parts of a plant is sprayed thoroughly with even mild detergent solutions such that all surfaces including leaves are covered, the detergent can easily dissolve the protective coating of cuticular wax on the leaves of the plant. Since these waxes have evolved to 1) prevent leaves from excessive water loss 2) provide a protective barrier to fungal and bacterial pathogens which denies them a pathway by which to enter the plant 3) acts as a sunscreen to protect the plant from harmful UV rays 4) act as a deterrent to some forms of insect herbivory. Some solutions will even dissolve cell membranes, causing death of cells.

If a detergent solution is used as a soil drench or makes its way into the soil as a consequence of over-spray or run-off, the effect on roots is immediate. Because one of the main function of roots is to absorb water and nutrients dissolved in water, they lack the waxy protective coating that benefits leaves. Detergent solution in contact with root cells can quickly dissolve the lipid membrane surrounding cells, killing the cells and inhibiting water/ nutrient uptake. Hair cells growing as appendages off larger roots do the lion’s share of assimilating water and nutrients, and these very delicate cells would be the first casualty of any detergent in the root zone.

There are widely available insecticidal soaps, designed to be used topically and made mixing potassium with only long chain fatty acids, carefully selected and specifically formulated to be safe for mammals & birds, death on most insects they contact, and most important, safe for plants. If you need something "soapy", it would be much better if you selected a product intended for use on plants than one intended for other purposes; and, it doesn't matter how many growers attest to the fact they've used it for a hundred years with no problems. If 50 million people practice an imprudent measure not recognizing it's imprudent, it's still imprudent.

While a water/ dishsoap solution might rid the plant of some insect pest, it can also leave the plant much more vulnerable to the next wave of insect predation, more vulnerable to fungal/ bacterial pathogens, at greater risk from solar damage, and limit the plant's ability to remain appropriately hydrated because of increased evaporative water loss.

Al