This is the seventh part of a series on spring tree insects. This article examines spruce spider mite, and whitefly.
Introduction
In spring, deciduous trees break dormancy, and resume the growing process. During this period, many insects become active, emerging from their overwintering sites to plague their hosts. The following describes some of the insects that may be commonly observed in spring, and how they can impact susceptible trees.
Spruce Spider Mite (Oligonychus ununguis)
Spruce spider mite is a small pest that infests numerous conifers. 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.
Treatment
- 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.
Whitefly (Aleyrodidae)
Whitefly is a small insect that infests a diverse array of plants. More than 1,500 species of whitefly have been discovered worldwide. Whiteflies comprise the family Aleyrodidae. The nymphs and adults are injurious to plants. They feed by extracting sap, and other plant liquids from new growth. The persistent feeding causes leaf discoloration, while stunting growth, and reducing crop and flower yield. Dense whitefly populations will gradually weaken host plants, rendering them more susceptible to disease pathogens, and other environmental stressors. Small plants may eventually succumb to infestation.
Hosts
Whitefly infests more than 250 ornamental and vegetable plants. Common hosts include astilbe, cabbage, chenille, citrus, columbine, cucumber, chrysanthemum, dicentra, eggplant, flowering maple, gerbera daisy, glory bower, grape, hibiscus, lavender, laventa, malva, monarda, mint, Martha Washington geranium, okra, poinsettia, potato, primula, salvia, scaevola, tomato, rosemary, sweet potato, verbena, and zinnia.
Symptoms of Infestation
Infested plants develop mottled leaves that turn yellow before dropping prematurely. Honeydew, an ooze that is excreted by the crawlers as they feed, may be observed on the branches and main trunk. Honeydew can encourage the development of sooty mold on the plant, which can smother growth, and inhibit the photosynthetic process. Ants are attracted to the honeydew, and can often be seen crawling on infested plants.
Treatment
- Crawlers and adults can be dislodged from infested plants using a strong stream of water.
- Whitefly has several natural predators, which are available for commercial use. Ladybugs and lacewing larvae feed on the eggs. Encarsia formosa and Eremocerus sp. are small parasitic wasps that eradicate the nymphs and pupae. The adult wasps lay eggs in the whitefly nymphs. As the eggs hatch, the larvae consume the nymphs from within, effectively killing them.
- Beauvaria bassiania is a common fungus that can be employed to control whitefly populations. The fungal spores penetrate the insect’s cuticle, causing it to become infected. Once the fungus has become established, it begins to expand within the insect, producing toxins that disrupt the insect’s immune system. Eventually, the insect will succumb to the infection. Do not apply fungicides in areas where Beauvaria bassiania is being used, as this can reduce the efficacy of the treatment.
- When ants are lured to a plant by the presence of honeydew, they will gather to protect the nymphs from their natural predators. This enables the nymphs to feed, and excrete more honeydew, which the ants readily consume. To effectively combat the nymphs, it is recommended that the ants be removed as well. There are several techniques that can be utilized to remove ants from infested plants. Infested plants can be doused with water, which will cause the ants to tumble to the ground. 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 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.
- Organic pesticides can be applied to infested plants to limit whitefly populations. Applications should begin as soon as the insects have been detected. If whitefly populations are dense, repeated applications may be required to establish control. Applications should be performed at seven to ten day intervals, as needed.
- Organic neem oil is safe for use on vegetables, fruit trees, and flowers. It can be administered during all stages of the insect’s life cycle. To ensure that the treatment is effective, thoroughly drench all leaf or flower surfaces.
- Horticultural oils can be used to smother the insects. They are effective at suppressing the eggs, nymphs, and adults.
- Botanical insecticides may be utilized to reduce whitefly populations. Applications should begin in early spring, during leaf expansion. Additional applications can be performed at seven to ten day intervals.
