Spider mites (family Tetranychidae) are common pests in agricultural and ornamental settings. These tiny arachnids are not insects but are related to spiders and ticks, often going unnoticed until damage is widespread. While they attack leaves, they do not chew tissue. They are sap-sucking pests that pierce the plant’s cellular structure to feed, leading to a decline in plant vigor and health.
How Spider Mites Damage Plants
Spider mites are piercing-sucking pests that extract the internal contents of individual plant cells. They use specialized, needle-like mouthparts, called stylets, to puncture the leaf’s outer layer (epidermis). Once the cell wall is breached, the mite sucks out chlorophyll, cell sap, and other contents from the internal mesophyll cells. This removal of chlorophyll, the pigment necessary for photosynthesis, is the primary source of damage. The destruction of these cells disrupts the plant’s ability to convert light energy into chemical energy, which can lead to stunted growth and reduced yield. As mites move, they leave behind a trail of empty, destroyed tissue that creates visible damage symptoms.
Identifying an Infestation
The most common sign of a spider mite problem is light stippling or speckling on the leaves, especially on the upper surface. These minute dots, typically yellow or pale white, mark the spots where mites have fed. As the infestation progresses, this speckling merges, causing the leaves to take on a yellowed, silvery, or bronzed discoloration. Heavily damaged foliage may become brittle and drop prematurely, causing defoliation.
A sign of severe infestation is the production of fine, silken webbing across the leaves and stems, which mites spin for protection and movement. To confirm their presence, inspect the undersides of the leaves, where these pests congregate and lay eggs. A simple method is to shake an affected leaf over a white piece of paper; the dislodged mites will be visible as tiny specks crawling on the surface.
Rapid Reproduction Cycle
The difficulty in managing spider mites stems from their accelerated reproduction cycle, which allows populations to explode rapidly under favorable conditions. The life cycle includes five stages:
- Egg
- Larva
- Protonymph
- Deutonymph
- Adult
Under optimal warm and dry conditions (around 80°F), the entire cycle from egg to mature, egg-laying adult can be completed in as little as five to seven days. A single female can lay up to 20 eggs per day, leading to hundreds of eggs over her two to four-week lifespan and resulting in numerous overlapping generations in a single season. Many common species lack a dormant phase, allowing them to reproduce continuously year-round in indoor environments like greenhouses.
Management Strategies
Effective management requires a multi-pronged approach that targets all stages of the life cycle. Cultural and physical control methods offer a non-chemical first line of defense, especially for small infestations. This includes forcefully washing the leaves with a jet of water, focusing on the undersides, to dislodge mites and break up webbing. Pruning and discarding heavily infested leaves also reduces the population. Since mites thrive in hot, dry conditions, increasing ambient humidity and ensuring plants are not drought-stressed creates an environment less favorable for proliferation.
For established problems, chemical and biological strategies are necessary. The use of “soft” pesticides like insecticidal soaps and horticultural oils is recommended, as they smother mites upon contact. Because these products do not affect eggs, repeated applications (often at five to seven-day intervals) are necessary to kill newly hatched mites before they reproduce. Biological control involves introducing natural enemies, such as predatory mites like Phytoseiulus persimilis, which consume eggs, nymphs, and adults. Successful management relies on the timely and repeated application of these methods, as the pests can quickly rebound and develop resistance.