This is the final article of a four part series on fall tree insects.
Introduction
In fall, while preparing for winter dormancy, many plants shed their leaves in a burst of color. During this period, certain tree insects can be easily identified. The following examines some of the most frequently observed fall tree insects, and how they impact their hosts.
Magnolia Scale (Neolecanium cornuparvum)
Magnolia scale (Neolecanium cornuparvum) is an insect that feeds exclusively on magnolia plants. It is one of the largest scale insects in the United States. The insect feeds by extracting plant liquids from the host’s leaves. The persistent feeding may result in extensive foliar dieback, and eventually, plant mortality.
Hosts
Magnolia scale feeds on magnolia plants. The insect is most partial to cucumbertree magnolia, lily magnolia, saucer magnolia, and star magnolia. It attacks other varieties of magnolia, albeit with less frequency.
Symptoms of Infestation
As the nymphs feed, they excrete large volumes of a sticky substance referred to as honeydew. When infested, a plant’s foliage will often become laden with honeydew. The honeydew may become blackened from the growth of a sooty mold fungus. When abundant, the honeydew can smother foliage, and inhibit plant growth. Honeydew is a vital resource for several insects. A bevy of flies, bees, wasps, yellowjackets, and ants feed on this sticky substance. Ants can be particularly problematic for infested plants. Once the ants have detected the honeydew, they will ascend the infested plant in order to feast on it. To encourage the production of additional honeydew, the ants will safeguard the nymphs from predators. This helps to increase scale populations.
As the nymphs feed, they sap liquids from the host plant. Heavily infested trees can incur significant foliar damage, resulting in an overall reduction in growth. Seed production may also be hampered. Twig and branch dieback is a common symptom of infestation. Infested twigs will appear enlarged, and may turn purple.
How to Treat Magnolia Scale
- Maintain plant vigor through sound cultural practices. Apply a layer of organic mulch around the base of magnolia plants to improve the soil quality, moderate the soil temperature, and retain soil moisture.
- Ensure that plants are sufficiently watered, especially during prolonged periods of drought.
- Horticultural oils may be utilized to smother the nymphs. Applications should begin in early spring, after the last frost has occurred, and prior to bud expansion.
- Insecticidal soaps are effective at eradicating the insects. Thorough coverage of infested foliage is required to ensure efficacy. Repeated applications may be necessary for dense populations.
- A variety of natural and synthetic insecticides are available for use to control the nymphs. Initial applications should be performed in late summer or early fall.
- Systemic insecticides can be applied as a drench treatment around the root zone of infested plants. The solution is absorbed into the roots, and transmitted throughout the plant, whereupon it is ingested by the nymphs. For the best result, treat the root zone in late winter, before the nymphs have become active.
- Magnolia scale has various natural enemies that help to minimize its populations. Some of the more notable enemies include lady beetles, predatory mites, and small parasitic wasps.
- When the nymphs are present, ants will tend to them. To prevent ants from accumulating on infested plants, place a band of sticky material around the trunk. Avoid placing any materials directly on the trunk, as they can have phytotoxic effects.
- Infested plants can be doused with water, which will dislodge the ants. Ants are also repelled by citrus and vinegar, which can be combined with water, and sprayed onto infested plants. The solution will mask the scent trails created by the ants, and act as a deterrent when honeydew is present.
- A mixture composed of boric acid powder, sugar, and water can be placed in several small containers, and distributed throughout the environment. The sugar will lure the ants to the tainted solution, which they will consume, and deliver to their queen. Once the queen has been exposed to the solution, she will expire, and the colony will collapse.
- Ant stakes or containerized bait may be placed on the ground to lure ants away from infested plants.
Spruce Spider Mite (Oligonychus ununguis)
Spruce spider mite is a small pest that infests numerous conifers. The insect is native to North America. It is one of the most destructive spider mites in the United States. Spruce spider mite is most active in spring and fall, when temperatures are cool. When conditions are favorable, spider mite populations can accumulate quickly, and devastate plants. Severely infested plants often experience widespread foliar dieback. If the spider mites are not tended to, some trees may succumb to infestation.
Hosts
Spruce spider mite infests a wide range of plants, including arbovitae, cedar, dawn redwood, hemlock, larch, juniper, pine, and other conifers. Dwarf Alberta, Colorado blue, Norway, and white spruce are the insect’s preferred hosts.
Symptoms of Infestation
Infested plants will assume a mottled or stippled appearance. Older needles will become discolored, turning a pale yellow. By mid-summer, most infested needles will turn red. By this point, many infested needles will drop prematurely. As the growing season progresses, discoloration may spread throughout the plant’s crown. Fine, silken webs can often be observed on infested needles from late spring to fall. Severely infested plants may experience significant foliar dieback. Trees that have been weakened by environmental stressors, diseases, or others infestations may collapse.
How to Treat Spruce Spider Mite
- Monitor plants that are vulnerable to infestation in early spring and early fall. If a plant’s foliage exhibits discoloration symptoms, perform a thorough inspection of the plant to determine if spruce spider mites are present. To do so, tap the suspected branch against a single sheet of white paper. If the branch is infested with mites, they will be dislodged by the sudden motion. If more than ten mites are observed on a branch, assess three to four more sections of the plant using the same method. If additional mites are discovered, apply a registered miticide to prevent them from breeding. Early spring is the most ideal period to administer miticides. A subsequent application can be performed in September if mite populations persist.
- Dousing infested plants with a strong stream of water can eliminate spider mites at all stages.
- Insecticidal soaps and horticultural oils can be utilized in spring to suppress the eggs and nymphs.
- Neem or pesticides registered for use in the control of spruce spider mite can be applied in fall to eradicate the adults.
- In fall, if infestations are severe, apply a dormant oil spray in mid-winter to kill the overwintering eggs.
- Maintain plant vigor through sound cultural practices. Ensure that plants are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around the base of susceptible plants to improve the soil quality, moderate the soil temperature, and retain soil moisture.
Photo courtesy of Paramecium CC-by-3.0
