An often discussed topic is the Wooly Apple Aphid (WAA). I asked an AI (Google Gemini) “What is the lifecycle of Wooly Apple Aphids?”.
I followed up with “Do they all leave the roots and move above ground or do some stay underground on the roots forever?”
The answer to the initial lifecycle question:
The lifecycle of the Woolly Apple Aphid (Eriosoma lanigerum) is quite complex and is often divided into two scenarios: the Complete Cycle (with an alternate host) and the Asexual Cycle (on apple only).
The latter, the Asexual Cycle on Apple , is the most common pattern in many commercial apple-growing regions where the alternate host is absent.1
Common Lifecycle (Asexual Cycle on Apple)
This is the cycle the aphid typically follows when it remains only on the apple tree, which is common outside of its native North American range or where the American Elm is scarce due to disease.2
1. Overwintering (Winter)
- Location: The aphid overwinters as wingless nymphs (first-instar “crawlers”) or adults.
- Where: They hide in sheltered locations, most commonly on the roots below ground or in cracks, under loose bark, or in pruning scars on the aerial parts of the tree.3 At this stage, they are not covered with the white “wool.”
2. Spring Migration and Colony Establishment (Spring)
- Activity: Overwintered nymphs become active in late March or April.4
- Movement: They begin to move, and crawlers often migrate up the trunk from the root colonies to the aerial parts of the tree.5
- Reproduction: They develop into wingless females (called stem mothers ) who reproduce asexually (parthenogenesis) by giving live birth to more wingless nymphs.6
- Appearance: They begin to secrete the conspicuous white, waxy “wool” that gives them their name, forming dense, cottony colonies on branches and suckers, often near wounds or pruning cuts.7
3. Summer Generations and Dispersal (Summer)8
- Rapid Growth: Multiple generations (8–12 per year in some regions) of wingless females are produced rapidly, greatly increasing the colony size on the shoots and branches.9
- Winged Forms: As populations peak or conditions change (typically around July/August), some nymphs develop into winged females (alates). These winged forms fly short distances to infest other apple trees, spreading the infestation.10
- Down Migration: Some wingless nymphs may also migrate down the trunk in the summer and fall to establish or reinforce root colonies , which are more damaging to young trees.11
4. Preparation for Winter (Autumn)
- Reproduction Slows: Breeding slows down as temperatures drop.
- Overwintering: Nymphs and adults move into their overwintering locations on the roots or protected bark crevices to survive the winter and restart the cycle the following spring.
The Complete Life Cycle (Native North America)
In its native range where the American Elm (Ulmus americana ) is present, the aphid alternates between two different hosts:12
Stage Host Plant Reproduction Role Overwinter American Elm Sexual Egg Eggs overwinter in bark crevices. Spring/Summer American Elm Asexual Live Birth Multiple generations form a rosetted leaf gall. The final generation is winged. Late Spring Migration Apple Tree N/A Winged females fly to the apple tree. Summer Apple Tree Asexual Live Birth Multiple generations of wingless aphids feed on bark and roots. Fall Migration American Elm N/A Winged females and males fly back to the elm. Fall American Elm Sexual Wingless males and females are produced, mate, and the female lays the single overwintering egg.
The answer to the underground question:
No, some Woolly Apple Aphids (WAA) stay underground on the roots year-round in apple-only environments, rather than all of them migrating above ground.
Root colonies are a critical component of WAA infestation, especially in areas where the American Elm (the alternate host) is absent.
Root vs. Aerial Colonies
The WAA population is typically split into two interlinked colonies on the apple tree:
- Edaphic (Root) Colonies: These are colonies established on the roots below the soil surface. They are the most damaging to the tree, as their feeding causes galls (swollen growths) that disrupt the plant’s ability to take up nutrients and water. These root colonies can persist and feed continuously throughout the year, especially in milder climates, and are the primary overwintering site.
- Aerial (Above-Ground) Colonies: These colonies form on the trunk, branches, and new growth, typically around wounds or pruning scars. They are most noticeable in the spring and summer due to their distinctive white, waxy “wool” covering.
The Migration Cycle
Movement primarily happens via the first-instar nymphs , known as “crawlers,” which are highly mobile:
- Upward Migration (Spring): Crawlers emerge from the overwintering root colonies and migrate up the trunk to establish the new aerial colonies on the shoots and branches. This is the main source of the visible, spring-time infestation.
- Downward Migration (Summer/Fall): As aerial colonies become crowded or the plant tissue hardens, crawlers will move down the trunk to reinforce the root colonies, where they will overwinter.
The root population remains a constant reservoir, continuously feeding and serving as a persistent source for re-infesting the canopy each year.
The second answer is a little concerning since I’ve discovered them on the roots of multiple of my trees. Ones which have died or been broken by a deer or similar. I’ve dug down to examine and more often than not find white “fuzz” all over the roots. Hopefully the cycle can be broken via an insecticide, applied to the limbs prior to their “upward migration” and subsequent feeding. I realize they’re hard to kill once established because the “fuzz” shields them from sprayed insecticides.
For the ones which are persistently underground, I have a few other weak/spindly trees I suspect may be suffering from this. I might apply a systemic insecticide around them this spring. They’re small enough it’s unlikely they would fruit that season anyway.
Thoughts or suggestions?