what insects does marigolds block
2. Spray the marigolds with the insecticide until all plant surfaces are wet. This will control sap-sucking insects such as aphids, thrips and spidermites. Repeat the treatment at 7 to 10 day intervals as needed. Kill chewing insects -- such as leaf-eating caterpillars -- by liberally spraying the marigold with a mixture of 4 teaspoons of Bacillus thuringiensis mixed with 1 gallon of water.