The webspinning two-spotted spider mite occasionally makes their home on bougainvillea. To the naked eye, spider mites look like tiny moving dots. Adult females, the largest forms, are less than 1/20 inch long. Spider mites live in colonies, mostly on the under-surfaces of leaves. The names “spider mite” and “webspinning mite” come from the silk webbing most species produce on infested leaves. The presence of webbing is an easy way to distinguish them from all other types of mites. Mites cause damage by sucking cell contents from leaves. A small number of mites is not usually reason for concern, but very high populations—levels high enough to show visible damage to leaves—can be damaging to plants. At first, the damage shows up as a stippling of light dots on the leaves; sometimes the leaves take on a bronze color. As feeding continues, the leaves turn yellow and drop off. Often leaves, twigs, and fruit are covered with large amounts of webbing. Damage is usually worse when compounded by water stress. Check the undersides of leaves for mites, their eggs, and webbing; you will need a hand lens to identify them. To observe them more closely, shake a few off the leaf surface onto a white sheet of paper. Once disturbed, they will move around rapidly. Be sure mites are present before you treat. Sometimes the mites will be gone by the time you notice the damage; plants will often recover after mites have left.
If a treatment for mites is necessary, use selective materials, preferably insecticidal soap or insecticidal oil. Petroleum-based horticultural oils or neem oils are both acceptable. Oils and soaps must contact mites to kill them so excellent coverage, especially on the undersides of leaves, is essential and repeat applications may be required. Mid-season washing with water to remove dust may help prevent serious late-season mite infestations. Regular, forceful spraying of plants with water will often reduce spider mite numbers adequately. Be sure to get good coverage, especially on the undersides of leaves.
Spider mites frequently become a problem after the application of insecticides. Such outbreaks are commonly a result of the insecticide killing off the natural enemies of the mites, but also occur when certain insecticides stimulate mite reproduction. Naturally controlling mites is the best method.
Cleaning around the plant is your best solution. Like wood borers, leaf miners are difficult to control as they are protected from insecticide sprays and plant defenses by feeding within the tissues of the leaves themselves. Some leaf miners can be killed by systemic pesticides (a type of pesticide that moves inside a plant following absorption by the plant), but many breeds are still immune to the effects of pesticide.
Cleaning around the plants. Debris tends to collect at the base of plants, and this is where the adults of the leaf miner larvae lay their eggs. Some leaf mining larva may also “winter over” in this debris. Removing leaves and other debris from around plants is an excellent method for controlling them.
Weeding provides an alternate food source for leaf miners, so areas around plants should be weeded and mulched.
Do not use contact pesticides. Since the leaf miner is inside the leaf, contact poisons cannot reach it, and therefore cannot kill it. Additionally, leaf mining insect larvae quickly become resistant to contact pesticides.