If every year they destroy your plants in that bed, then yep, sure, they were just waiting there under the row covers. In fact, the row covers told them what to pay attention to once they hatched. Covering a bed that has hosted vine borers without trying to get rid of the pupae resident in the soil is not a good plan. Getting rid of pupae isn't easy, by the way.

+1

If you don't want to sift/screen the soil for SVB larvae then planting something that won't attrract the SVB for a year or two is in order.

btw - the row cover in the pic doesn't look like it is sealed down correctly either.

+2

Sorry to be the bearer of bad tidings, but the bad news is: if you had borers in that bed last year, consider the soil in that area infested - because it probably is. In that case, moths will emerge from beneath the cover, rendering it useless. Or worse than useless, since the moths will be trapped long enough to infect all plants under the cover... which is probably what happened.

As for the plants already infected; while it is possible to cut out the grubs & save the plant, success is far from a sure thing - and you don't want even more pupae getting into your soil. Personally, I would recommend pulling all of those plants, and destroying them in such a way as to kill the larvae. Then plant something else in that space next year, which should get rid of any larvae already in the soil.

I agree with Gumby, regarding the apparent looseness of the row cover. It must be tightly sealed to be effective. The best way is to bury all of the edges.