I'm not willing to use the Sevin:
Carbaryl (active ingredient in Sevin) can produce adverse effects in humans by skin contact, inhalation or ingestion. Athough, it may cause minor skin and eye irritation, carbaryl does not appear to be a significant chronic health risk to humans at or below occupational levels. In humans, acute effects of carbaryl exposure include headaches, nausea, incoordination, and difficulty breathing. Carbaryl can cause a variety of behavioral effects, some of which are relatively long-term. It also suppresses several functions of the immune system. Exposure to carbaryl has been associated with a higher incidence of the cancer non-Hodgkin's lymphoma in farmers and brain cancer in children
Carbaryl is acutely toxic to fish. Carbaryl is acutely toxic to birds, although the dose required to kill most species is greater than that required to kill mammals, fish or insects. Carbaryl also adversely affects birds at lower doses by reducing the population of insects and aquatic invertebrates that the birds feed on. Unhatched and young birds appear to be particularly sensitive to carbaryl exposure. Earthworms are sensitive to small amounts of carbaryl in soil. In field studies, carbaryl treatment reduced earthworm populations by between 50 and 90 percent. Follow-up studies showed that populations took five to twelve months to recover and that the rate at which mineral soil was incorporated into thatch was significantly impaired during this period. It is toxic to frogs, shrimp, crabs, clams, snails, some aquatic insects, and many pond living creatures. Carbaryl is highly toxic to honey bees, certain beneficial insects such as lady beetles, and parasitic wasps and bees.
While insecticides are not usually assumed to have adverse effects on plants, carbaryl's use as a plant growth regulator (chemical thinning agent) makes effects on other plants unsurprising. It has been shown to decrease germination success, inhibit seedling growth, reduce photosynthesis, and reduce nitrogen fixation. Note that several different people have written in to helpfulgardener to say that their gardens died after being sprayed with Sevin.
Fate in the environment:
Because of its chemical characteristics, the U.S. Environmental Protection Agency (EPA) identified carbaryl as one of the pesticides with most potential to leach into groundwater. It has been found in groundwater in California, Missouri, New York, Rhode Island, Virginia, and Wisconsin. Carbaryl was the most commonly detected carbamate insecticide in the 1991 U.S. Food and Drug Administration pesticide residue monitoring program; it was the tenth most commonly detected pesticide. It was also one of eight pesticides detected in baby food samples . It has been banned in United Kingdom, Denmark, Australia, Germany, Sweden, Iran and Angola and other places. See https://www.helpfulgardener.com/forum/vi ... 11&t=57653
And even if you are willing to use it, not sure how well it will work. The trouble with the borers is the adult flies in, lays her eggs at the base of the stem. She may or may not be bothered by the residual insecticide there, since she doesn't crawl over it and doesn't ingest any or stay long. But even if it kills her after she lays the eggs , that doesn't matter. She was probably going to die soon anyway. So then the question is will the residual insecticide harm the eggs? I don't know the answer, but eggs are inside the shell and are a lot harder to kill than bugs. Then the eggs hatch into larvae which drill into the stem. So is residual insecticide going to kill the larvae while they drill through it? Again I don't know, but it is a brief window of opportunity. After that, they are inside and nothing you do to the outside will matter much.
I have tried keeping the base of the stem covered with soil; that did not save my plants. They say you can spray the stem with kaolin clay which acts as a protective barrier. I have not tried that. As I said, I just gave up - too heart breaking to have a big beautiful plant produce the first zucchini or two and then just collapse and die. I know other gardeners here in town who have also given up on them.