Twisted needle symptom. Courtesy of Tracey Olson, PDA
Mindarus abietinus Koch
- All species of true fir, especially Fraser and balsam
- Rarely found on some spruce and juniper
Symptoms and Signs
- Curled, twisted needles on current year’s growth
- Stunted needles
- Black sooty mold and presence of stinging insects
Causes of Similar Symptoms
- Pesticide use
- Soft scales
- Other aphids
Balsam twig aphids are tiny, soft-bodied insects with piercing-sucking mouthparts. Most stages are pale bluish green and some may have powdery, wax strands adhering to the body. The second generation is the only one that produces winged forms; all other generations have only wingless adults. The largest stage, the “stem mother,” occurs in the first generation and at maturity is 1⁄25–1⁄13 inch (1–2 mm) long.
Eggs are small ovals coated with waxy rods that have sloughed off from the underside of the female. Initially, eggs are pale tan, but they darken with age and by spring appear to be silvery black. With the white, waxy rods and almost black eggs, it is relatively easy to spot eggs using a hand lens of at least 15X magnification.
Calendar of Activities
Biology and Life Cycle
The balsam twig aphid has a complex life cycle with three to four generations occurring per year. Most of the year is spent in the silvery egg stage on twigs of the host tree (Figure 1). In early April, prior to bud break, the overwintering egg hatches into a small nymph (Figure 2). This nymph feeds for a period of time on the underside of last year’s needles before molting into the wingless stem mother. As buds are swelling and just beginning to open, the stem mother moves to the bud (Figure 3) and gives live birth to up to 70 second-generation nymphs. No mating or eggs are involved; the nymphs are clones of the stem mother and capable of feeding immediately.
Figure 1. Silvery balsam twig aphid egg at the needle’s base on the underside of a twig. Courtesy of Rayanne D. Lehman, PDA
Figure 2. Newly hatched nymph feeding on needles and secreting “honeydew.” Courtesy of Rayanne D. Lehman, PDA
Figure 3. Stem mother on needle bud. Courtesy of Rayanne D. Lehman, PDA
This second generation is the most damaging (Figure 4). Perfectly synchronized with bud break, the nymphs begin to feed on the elongating needles (Figure 5), causing distortion and stunting (Figure 6). Within a short time, they have matured and eventually produce live, either wingless or winged females. If wingless forms are produced, they continue to feed on the same tree. Although the winged females are weak fliers, they are capable of short flights or becoming airborne to be carried by air currents to new hosts.
Figure 4. Balsam twig aphid nymphs. Courtesy of Rayanne D. Lehman, PDA
Figure 5. Nymphs (circled) feeding in a barely opened bud. Courtesy of Sandy Gardosik, PDA
Figure 6. Needle twisting or kinking evident of balsam twig aphid damage. Courtesy of Cathy Thomas, PDA
When the winged females reach the new host or new stem on same tree, they produce both wingless females and males (Figure 7). This last generation is the only one with males that mate with the females. Mated females deposit one or two overwintering eggs close to the buds before dying. This is also the only time in the life cycle that eggs are produced.
Figure 7. Winged balsam twig aphid female. Courtesy of Rayanne D. Lehman, PDA
Monitoring and Management Strategies
- Avoid planting at lower elevations and on slopes where aphid populations tend to be higher.
- Plant species that have delayed bud break. Balsam fir tends to break bud early and is most severely infested; Fraser is next to break bud; Canaan generally breaks bud last.
- Do not buy infested trees or nursery stock.
- Between July and March, assess damage levels from the previous year’s infestation. It is most important to control this pest during the last 2 years before harvest in order to produce 2 years of good, straight growth. On young trees, excessive damage will stunt overall growth.
- Do not apply a nitrogen fertilizer before bud break to a previously infested area. This tends to increase aphid populations.
- Look for overwintering eggs and tag trees to monitor for egg hatch by the end of March.
- Growing degree days: Egg hatch occurs at 30–100 GDDs.
- Begin monitoring for egg hatch by April 1 in southern counties. Using a hand lens, examine the underside of last year’s needles for single, blue-green nymphs. These nymphs are usually within 2 inches of the bud and often have a large droplet of clear honeydew on their tail end.
- Encourage natural predators and parasitoids.
- Scout for aphids by beating 10 inches of foliage over a piece of dark paper. Select 15 trees per acre (similar age, size, and location) and sample two sides of each tree. Count aphids and predators found. In April or early May, it may be necessary to apply chemical control if more than three aphids per tree are found.
- As bud break begins, look at the opening buds to find the stem mothers and second generation nymphs.
- When vegetative cones are produced on trees, break open the cones and look for nymphs that have migrated here to feed.
- Observe presence of lady beetles since several species are important predators.
- Observe presence of stinging insects as new growth elongates. The bees and wasps are attracted to the honeydew produced by the feeding aphids.
- At the end of the season, update records and evaluate results.
- Balsam twig aphids have numerous naturally occurring predators, including yellow jackets, lacewings, earwigs, lady beetles and their larvae (Figure 8), assassin bugs, ants, big-eyed bugs, predatory thrips, hover fly larvae, syrphid flies, and predaceous midges. The introduced multicolored Asian lady beetle is an excellent aphid predator in the adult and larval stage. Parasitoids of balsam twig aphids are Aphidius wasps. Refer to Appendix B: Biological Controls Photo Chart for pictures.
Figure 8. Larva of multicolored Asian lady beetle. Courtesy of Cathy Thomas, PDA
- Pick and destroy young cones in spring during aphid activity. Since pesticides will not reach the aphids inside cones, this practice can be time consuming but very beneficial and will positively affect the aesthetics of the tree.
- No recommendations are available at this time.
- All pesticide intervention should occur after egg hatch but before bud break; after bud break, aphids are protected by new growth and some damage has already occurred. Dormant oil will kill aphids but will have limited effect on overwintering eggs and often causes damage to elongating needles.
- Purchase and plant pest-free nursery stock from a reputable company.