Spruce Spider Mite

Spruce spider mite (Oligonychus ununguis) is an arachnid pest notorious for causing extensive damage to coniferous trees, particularly spruce trees. This mite is microscopic and a member of the Tetranychidae family, closely related to spiders, ticks, and other mites.

Host Plants

The primary host for the spruce spider mite is spruce trees, particularly the Colorado blue spruce. However, it can also infest other coniferous trees, including juniper, arborvitae, fir, hemlock, and pine trees.

Regions impacted

The spruce spider mite is a widespread pest found across the United States and Canada. It is one of the most destructive spider mites in the US.

Description

Adult spruce spider mites are oval and generally measure less than 1/50th of an inch in length, making them very difficult to see without magnification. They have eight legs, and their body color varies from dark green to dark red or brown, often depending on the host plant they’ve been feeding on and their life stage.

The nymphs, which are immature forms of the mite, are similar in appearance to the adults but are smaller in size and lighter in color.

One distinctive characteristic of the spruce spider mite is its ability to produce silk-like webbing. This webbing can sometimes be spotted on the needles and branches of infested trees, especially with a large mite population. While this webbing is not as extensive as other spider mites, it can help confirm their presence and provide some protection to the mites from predators and environmental conditions.

Life Cycle

The life cycle of the spruce spider mite (consists of four primary stages: egg, larva, nymph, and adult. Here’s an overview of the process:

Egg: The life cycle of spruce spider mites begins with overwintering females laying eggs on host trees, particularly on the undersides of needles, when temperatures begin to rise in spring. These spherical, reddish-colored eggs are tiny and difficult to spot without a magnifying glass.

Larva: Once the eggs hatch, they emerge as six-legged larvae. Larvae feed on the host tree by piercing the foliage and sucking out the plant juices, which can lead to the characteristic stippling or speckling appearance on the leaves.

Nymph: After feeding for some time, larvae molt and enter the nymphal stage. Nymphs resemble miniature adults but are not yet sexually mature. The nymph stage itself consists of two substages, known as the protonymph and deutonymph stages, each separated by a molt.

Adult: Once the deutonymphs molt, they become adults. They continue to feed on the plant, causing further damage.

Interestingly, the spruce spider mite has a reverse dormancy pattern compared to many other mites and pests. They are most active during the cooler spring and fall seasons, slow down during the hot summer months, and enter dormancy or hibernate during the winter.

The entire life cycle, from egg to adult, can take as little as two weeks under optimal conditions, but this may vary based on temperature and other environmental factors. As such, multiple generations can overlap within a year, making infestations potentially severe if not properly managed.

Damage and Detection

Damage

As their name suggests, spruce spider mites primarily attack spruce trees but also infest other conifers like fir, hemlock, pine, and arborvitae. Their feeding activity can lead to considerable damage that compromises the health and aesthetics of the trees.

Spruce spider mites pierce plant tissues with their mouthparts and suck out the fluids within. This feeding activity disrupts the process of photosynthesis and leads to a distinct damage pattern:

• Needles of the affected trees first exhibit tiny yellow or white specks, a symptom known as stippling. Over time, this stippling coalesces, leading to a more general yellow or bronze discoloration of the needles.
• As damage progresses, the needles may turn rusty-brown or reddish, then dry up and drop off the tree prematurely.
• When mite populations are particularly high, the infestation can lead to a general thinning of the tree, dieback of branches, and potentially the tree’s death if left unchecked.

Detection

Detecting spruce spider mites can be challenging due to their small size. However, some signs can point toward their presence:

• As mentioned above, the initial signs of mite activity are the stippling on needles, followed by a yellow or bronze discoloration.
• Another clue to their presence is the production of fine silk webbing on the plant, similar to spider webs. This webbing can be particularly noticeable when it collects dust or water droplets.
• A simple and effective technique for detection is to place a white piece of paper or cloth underneath the branches and then shake or tap them. The mites, if present, will fall onto the paper and can be seen as tiny, moving specks.
• You may also spot their eggs, tiny and round, on the undersides of needles, especially in early spring.

Prompt detection is key to managing spruce spider mite infestations effectively. If you suspect an infestation, consider contacting a local cooperative extension service or a pest management professional for confirmation and guidance on treatment options.

Prevention and Control

Preventing and controlling spruce spider mites involves a blend of cultural, biological, and chemical strategies:

Cultural Control:

• Regular Inspection: Inspect your trees regularly, particularly in spring and fall when mites are most active. Look for signs of stippling or yellowing of needles, and perform the white paper test to confirm their presence.
• Maintain Plant Health: Healthy trees are less susceptible to mite infestation. Ensure that your trees get the right amount of water and are not under stress from a nutrient deficiency or environmental extremes.
• Prune Infested Branches: If you detect mite activity on a particular branch early, pruning it can prevent the mites from spreading to the rest of the tree.
• Wash the mites and eggs off the foliage with a strong stream of water: The best time to do this is in the early morning. Mites are less active during the morning’s cooler temperatures, making it harder for them to return to the tree. Spray all parts of the tree, paying special attention to the undersides of the needles where the mites and eggs are most likely to be found.
• Avoid Broad-Spectrum Insecticides: These can kill beneficial insects that naturally control mite populations, potentially leading to a surge in mite numbers.

Biological Control:

• Encourage Beneficial Insects: Predatory mites, ladybugs, and lacewings, minute pirate bugs are natural enemies of spruce spider mites. By encouraging these beneficial insects in your landscape, you can help keep mite populations in check.

Chemical Control:

• Insecticidal Soaps or Horticultural Oil: Both of these products can be effective in managing spruce spider mites. They work by suffocating the mites and their eggs. It’s important to ensure thorough coverage of the tree when using these products, particularly focusing on the undersides of the needles. Also, keep in mind that these products only kill on contact and have no residual effect, so multiple applications may be necessary.
• Neem or Pyrethroid-based Pesticide: Neem oil is a natural insecticide that can disrupt the life cycle of mites. Pyrethroids are synthetic pesticides that kill on contact and have some residual activity. Both could be used later in the season when mite populations typically decrease. However, they should be used judiciously to avoid harming beneficial insects.
• Dormant Oil Spray in Mid-Winter: If you had a severe mite infestation in the fall, a dormant oil spray in mid-winter can be a good strategy to control overwintering eggs. Dormant oils are highly refined petroleum products or natural plant-based oils that are used to smother pests. Apply the oil when the temperature is above freezing but below 60 degrees Fahrenheit.
• Miticides: If the infestation is severe and other control methods have not been effective, you might need to consider using a miticide (also known as an acaricide). These are pesticides specifically designed to kill mites.
• Timing and Application: Apply miticides in spring or fall when the mites are most active. Remember to thoroughly spray the undersides of needles where mites and their eggs are present.
• Follow Instructions: Always follow the product’s instructions for the best and safest results.

Remember that control of spruce spider mites is not about eradication but about managing their populations to a level where they do not cause significant damage. Sometimes, minor mite damage can be tolerated, particularly if the tree is otherwise healthy. Always aim for a balanced approach that maintains the health of your trees while preserving the ecological balance in your garden or landscape.