Panaphis juglandis

(Goeze, 1778)

Walnut Aphid, Large Walnut Aphid

Panaphis juglandis, the large walnut , is a phloem-feeding pest of walnut trees (Juglans regia). It colonizes exclusively the upper (adaxial) surface of walnut compound leaves, forming dense colonies along the main vascular of the outer leaflets. Native to the Old World (Central Asia and southeastern Europe), it has spread across Europe and was introduced to California in the early 20th century. The reduces tree vigor, nut yield, and quality, and excretes honeydew that promotes growth. It forms mutualistic relationships with species that tend and protect colonies from .

Panaphis juglandis by Geert Orye. Used under a CC BY 2.0 license.

Panaphis juglandis and Crematogaster scutellaris 05 by Syrio. Used under a CC BY-SA 4.0 license.

Panaphis juglandis Piazzo by Syrio. Used under a CC BY-SA 4.0 license.

Pronunciation

How to pronounce Panaphis juglandis: /ˈpænəfɪs d͡ʒʌˈɡlændɪs/

These audio files are automatically generated. While they are not always 100% accurate, they are a good starting point.

Identification

Distinguished from the small walnut (Chromaphis juglandicola) by its larger body size and exclusive feeding on the upper leaf surface; C. juglandicola feeds only on the lower leaf surface. Forms characteristic colonies along the main vascular , particularly on the first three external leaflets of compound leaves. Darkening of the leaf midrib occurs shortly after and persists after colonies depart.

Images

Habitat

Urban and suburban environments with walnut trees, including street-side locations with strong anthropogenic disturbance and home gardens. Temperate climate zones. Colonizes exclusively the upper surface of walnut compound leaves.

Distribution

Native to the Old World with homelands in Central Asia and southeastern Europe. Widespread across Europe from Spain, Italy, and Serbia to Denmark, Sweden, and Poland. Introduced to California at the beginning of the 20th century. Present in Yugoslavia, Serbia, Albania, and Austria.

Seasonality

Active from spring through early autumn. First specimens appear in April–May, with earlier in street-side locations compared to home gardens. Peak typically occurs in June, with activity continuing through August–September depending on temperature. Two peaks possible in some years when temperatures decrease in late summer.

Diet

Phloem feeder on walnut (Juglans regia). Feeds exclusively on the upper (adaxial) surface of walnut compound leaves.

Host Associations

Juglans regia L. - primary common walnut; sole documented in detailed studies

Life Cycle

Development begins on external leaflets (positions 1–4), then spreads to all leaflets as increase. Both alatae (winged females) and apterae (wingless forms) present; alatae females observed in May–June. Population development influenced by weather: temperatures of 15–20°C with average rainfall enhance populations; temperatures above 25°C with intensive rainfall decrease populations; temperatures below 15°C cause population decline.

Behavior

Forms dense colonies along the main vascular on the upper leaf surface. Engages in mutualistic relationships with that tend colonies and protect against . Ant defensive vary by species: some ants fight or attempt rescue of grabbed aphids, while others flee when confronted by lady beetle predators.

Ecological Role

Pest of walnut that reduces tree vigor, nut size, yield, and quality. Excretes copious honeydew that supports growth of black fungi, reducing light penetration and photosynthetic capacity and predisposing nuts to sunburn and kernel quality loss. Serves as prey for lady beetles (Coccinellidae) including Harmonia axyridis and Adalia bipunctata. Mutualist partner for various (Camponotus piceus, C. vagus, Crematogaster scutellaris, Dolichoderus quadripunctatus, Lasius emarginatus) that tend colonies and provide protection from natural enemies.

Human Relevance

Significant pest of walnut orchards and urban walnut trees. High (>15 aphids per leaflet for at least 14 days) reduce current season nut quality and the following season's crop. Honeydew promotes that degrades nut quality and tree health. Subject to research involving predatory lady beetles and dynamics.

Similar Taxa

Chromaphis juglandicolaBoth are walnut-feeding aphids, but C. juglandicola is smaller and feeds exclusively on the lower leaf surface, while P. juglandis is larger and feeds only on the upper surface.