Soldier Aphids and the Emergence of Primitive Social Behavior

Aphids are commonly regarded as insects that gather densely on plants yet possess little ability to defend themselves. When predators such as ladybird larvae, hoverfly larvae, or parasitic wasps appear, many aphid species rely on ants for protection. Familiar examples include the cotton aphid (Aphis gossypii) and the green peach aphid (Myzus persicae). Although these insects often form dense colonies, their aggregations generally lack any sign of task specialization and therefore are not considered social insects.

However, certain aphid species have evolved specialized defensive individuals—soldier aphids—that protect their colonies. This represents a primitive form of social organization. As in many other social insects, these soldiers display aggressive behavior and often perform self-sacrificial actions that benefit the colony. Such evolutionary developments are known in species belonging to the families Pemphigidae and Hormaphididae. Two representative examples are the Japanese gall-forming aphid Nipponaphis monzeni and the bamboo aphid Pseudoregma alexanderi.

Cotton aphid（感謝Jim Baker提供）

Green peach aphid（圖片來源：Alexis，採用 CC BY 4.0 授權）

Nipponaphis monzeni

Many species in the family Pemphigidae induce plant galls that serve as shelters for their colonies. These structures function both as a defensive fortress and as a food source.

In the life cycle of Nipponaphis monzeni, winged sexual-producing females (alate sexuparae) fly to the plant Distylium racemosum. There they produce wingless males and sexual females. After mating, the females lay eggs, which hatch into first-instar fundatrices. These founding aphids release chemical signals that trigger the plant to form a gall. Once inside the gall, the fundatrix survives through the winter and begins reproducing parthenogenetically the following year, producing numerous nymphs.

The gall forms a sealed chamber in which the aphid colony feeds on plant sap. Early nymphs appear white and covered with long hairs. As time passes the gall enlarges, and some first-instar nymphs develop into specialized soldier aphids. These soldiers are responsible solely for colony defense. When the gall wall is damaged or predators attack, the soldiers sacrifice themselves by releasing body fluids that seal the breach, protecting the rest of the colony. These soldier aphids remain permanently in the first-instar stage and never develop into adults.

When winged adults (alates) eventually develop, the gall opens and roughly 600 to 800 aphids emerge. They migrate to secondary host plants such as Quercus glauca and Castanopsis sieboldii, settling on branches and the undersides of leaves. At this stage the aphids form open colonies rather than living inside galls. The wingless nymphs cling tightly to branches and appear yellow-brown or pale brown. Unlike the gall stage, these colonies do not produce soldier aphids. Their defense against predators is still poorly understood, but it may involve camouflage, the protective effect of living in groups, and rapid reproduction that compensates for predation. Later, winged females are produced, allowing the life cycle to begin again.

Nipponaphis monzeni inside a gall on its primary host plant（感謝Mayako Kutsukake提供）

Gall of Nipponaphis monzeni（圖片來源：Utako Kurosu、Shigeyuki Aoki，採用 CC BY 3.0 授權）

Nipponaphis monzeni on its secondary host plant（圖片來源：Utako Kurosu、Shigeyuki Aoki，採用 CC BY 3.0 授權）

Pseudoregma alexanderi

The aphid Pseudoregma alexanderi lives on the giant bamboo Dendrocalamus latiflorus. Unlike Nipponaphis monzeni, it does not build galls or other protective structures. Instead, it forms exposed colonies on the host plant.

Within these colonies, some individuals differentiate into soldier aphids. These soldiers are also first-instar nymphs and never develop into adults, meaning they permanently lack reproductive ability. Compared with ordinary reproductive nymphs, soldier aphids are larger and possess hardened exoskeletons, thickened forelegs, and well-developed frontal horns that serve as weapons against predators.

Most adults in the colony are wingless females that reproduce parthenogenetically, producing the next generation. During spring, some individuals develop into winged forms capable of dispersing to other bamboo plants, allowing the colony to spread.

Pseudoregma alexanderi, the larger individual on the left is a soldier（圖片來源：Keigo Uematsu et al，採用 CC BY-NC-ND 4.0 授權）

Dendrocalamus latiflorus（圖片來源：Daderot，CC0 1.0 公共領域）

Author: Shui-Ye You

Reference:

1. Kurosu U and Aoki S. (2009). Extremely Long-Closed Galls of a Social Aphid. Psyche: A Journal of Entomology.

2. Uematsu K et al. (2023). Eusocial evolution without a nest: kin structure of social aphids forming open colonies on bamboo. Springer Nature.