I'm sorry to hear you had this problem. It's always disappointing to lose a crop. The grubs you have described are the larval form of the onion fly, and are just called onion maggots. The flies actually lay their eggs in the spring near any plants in the onion family. When the maggots hatch, they go into the soil and burrow into the plant. They grow there until they are so big they destroy the plant. By then the maggots are ready to go on to the next stage of their life. These pests have been common in the UK, Canada, and parts of the States.
It takes several steps to control them. Make sure you destroy every bit of the plants because pieces of them left in the ground will attract the flies next year. Also, manure draws them in. When you plant, work in some diatomaceous earth. This is an all-natural way to kill insects. It consists of the fossilized remains of tiny seas creatures. Their edges are sharp to insects, and lead to them being dehydrated. Each time you weed, put more diatomaceous earth at the base of your plants. The earth comes in several grades, and you'll want the one for gardening or pest control. It is sold in garden centers and nurseries, or you can buy it online:Diatomaceous earth
You can also use floating row covers over your plants to keep the flies out.
Some gardeners use the chemicals tefluthrin and chlorpyrifos in the soil and on the plants. Too much chlorpyrifos is harmful to your plants, so it has to be applied carefully. Better results are usually obtained with a combination of diatomaceous earth and floating row covers.
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