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For many gardeners, marigolds are a welcome addition to their flower and vegetable gardens. Often this is not because of their attractive orange and yellow blooms, but for their ability to repel insects. For this reason, people often mistakenly believe that marigolds are resistant to pests themselves. Although there are few insects that are problematic for the flower, it is not completely pest-free. Common marigold pests include snails and slugs, Japanese beetles, leafminers, and twospotted spider mites.
Snails and slugs feed upon a variety of plants, and this includes marigolds. Their primary method of feeding involves chewing up the leaves. Unlike other marigold pests, these hermaphroditic intruders are neither hard to find, quick, nor prodigious, which means that they can be removed from the property by hand, or easily destroyed. In addition, it is easy to track them by their slime trails. They can also be kept away from marigolds by the use of copper barriers which are embedded a few inches into the ground and rise four inches above the surface.
Japanese beetles are a common pest of both the African and American marigold. Adults destroy the appearance of the plant by devouring its foliage. They primarily eat during the day and consume nearly all of the leaf tissue, with the exception of the veins. These marigold pests can be managed by simply picking visible beetles from the leaves. Insecticides and beetle traps are also an option for eliminating them.
Twospotted spider mites are another species of marigold pests. With the largest mites being 0.05 inches (1.27 millimeters) in size, it is nearly impossible to spot them except with the use of a magnifying glass. They feed on marigolds by sucking out the chlorophyll from the plants' leaves. The damage can be identified by leaves turning bronze or developing yellow spots. These pests can be exterminated by the use of pesticides, or the introduction of ladybird beetles who prey upon them for food.
A leafminer is a type of flying insect that has a negative affect on the appearance of marigolds. The leafminer name comes from the "mines" or tunnels that are created on the leaves as the insect's larvae eat from the tissue. Fortunately, the mines created by these marigold pests are not severe enough to kill the plant or most leaves. In addition to the damage caused by the larva, adult leafminers puncture the marigold's leaves to feed. This results in unattractive brown spots on the surface in addition to the mines.