When brassica crops wilt unexplainably, the most likely cause is cabbage maggots. The first wave of these little root and stem feeders in early spring usually is the most destructive. By delaying planting until after yellow rocket (wild mustard) has bloomed, you can avoid these maggots.
If there are no brassicas (such as the yellow rocket) in the garden for the adult flies to lay their eggs around (which eventually hatch into those root-eating maggots), they'll move on. If you're direct-seeding your crops, wait a week after the peak of yellow rocket's bloom to sow. Wait about two weeks if you're putting out transplants.
One of the most destructive sweet corn pests in North America, corn earworms chew through the silks and into the tips of the ears, making an unattractive mess. Plant early maturing varieties as early as possible to avoid sharing your sweet corn. Corn planted much after that will probably be silking just as the corn earworm moths are looking to lay their eggs.
Consider starting your corn indoors in peat pots and then transplanting, pots and all. Corn won't germinate in cool soil, but it grows just fine in it.
Colorado Potato Beetle
Simply delay planting your potatoes for 7 to 14 days after the usual planting date in your area and you could defeat this defoliating pest! Adult Colorado potato beetles (CPBs) emerge from the soil in early spring. If they don't find any potato plants, they move on.
Just be sure you plant fast-maturing varieties so the tubers will be a good size when the second generation of beetles comes around in midsummer. A little leaf chewing by the beetles (who only eat the above-ground growth) at that point won't hurt your harvest.
To further protect your late-planted potatoes, build a trench trap when you plant your potatoes. Surround the bed with a 16-inch-deep vertical-sided trench lined with black plastic—the beetles will crawl to get to the potatoes, fall into the trench, and won't be able to escape.
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