Tuberolachnus salignus
Tuberolachnus salignus, showing dorsal tubercle
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hemiptera
Suborder:	Sternorrhyncha
Superfamily:	Aphidoidea
Family:	Aphididae
Genus:	Tuberolachnus
Species:	T. salignus
Binomial name
Tuberolachnus salignus Gmelin, 1790

Tuberolachnus salignus, or the giant willow aphid, is a species of aphid, in the genus Tuberolachnus. They are reputed to be the largest aphids, with a body length of up to 5.8mm. ^{[1] [2]} First described by Johann Friedrich Gmelin in 1790, it feeds on many species of willow (Salix species), and has one known host-specific parasite, Pauesia salignae. ^[2]

Tuberolachnus salignus does not require a male for reproduction. Only females have ever been recorded, which suggests that reproduction is parthenogenetic and all offspring produced are genetically identical clones. ^[3]

Description

Adults exhibit a pear-shaped body with dark brown/grayish coloration. ^[4] This aphid species has distinct black dots that form rows along its body. ^[4] Along the dorsal side of their abdomen, about 2/3 of the way down, Tuberolachnus salignus displays a defining single large conical tubercle or bump that resembles a thorn which can grow to 0.2-0.3 mm. ^{[5] [6]} Closer to the posterior and along the sides of the body are two smaller black cone structures which excrete honeydew. ^[4] Honeydew is a modified form of honey that looks dark in coloration and crystalizes when exposed to air. ^[7]

Tuberolachnus salignus reproduces solely through parthenogenesis so all aphid adults are female. ^{[5] [7] [8] [9]} Adults can be either winged or wingless. ^[4] The two pairs of wings are clear with a dark brown edge along the forewing. ^[4] When looking at Tuberolachnus salignus under a microscope the presence of numerous tiny hairs is apparent. ^{[4] [5]} This gives the abdomen a grayish-gold hue. ^{[4] [5]} Adult Tuberolachnus salignus has three pairs of legs that are primarily a dark brownish color, but smaller, red/orange sections are also present. ^{[4] [5]} They also have a pair of antennae that are less than half of the aphid’s body length. ^{[4] [5]}

Range

Natural Global Range

Tuberolachnus salignus is a cosmopolitan species. ^{[7] [10]} They exist anywhere that willow species grow naturally or are cultivated. ^{[7] [10]} The giant willow aphid originated in parts of Asia but has now spread to all parts of the globe (except Antarctica). ^{[7] [10]} There is no known exact place of origin for this aphid species, but it is believed to be somewhere within Japan and Korea. ^{[9] [11]} The only known parasitoid of this species is endemic to Japan, leading to the belief that this may be the place of their origin. ^{[9] [11]}

Tuberolachnus salignus is now found in all parts of New Zealand where willow species are present. ^{[9] [11]} It only recently made it to New Zealand, with the first confirmed sighting in December of 2013 in Auckland. ^{[9] [11]}

Habitat

Tuberolachnus salignus is almost entirely dependent on willow trees. ^{[9] [11]} They live on the bark of these trees and feed on their sap. ^{[9] [11]} These giant aphids survive year-round with the most activity being in late summer and early fall. ^{[5] [9] [11]} Their population numbers are known to be largely weather and temperature related. ^[12] Following instances of large amounts of rain, Tuberolachnus salignus populations have been known to increase rapidly. ^[3]

Ecology

Life cycle/Phenology

Tuberolachnus salignus has a generation period of 2–3 weeks, which is largely dependent on temperature. ^{[6] [3]} Once hatched, the aphid larvae go through four stages of development until they reach their adult forms. ^{[5] [13]} Ideal temperatures for Tuberolachnus salignus generation range from 17.5–22.5 degrees Celsius. ^[6] It takes two weeks for nymphs to become adults, with adults living an additional five weeks after that. ^[5] Mature aphids can produce anywhere from 35–71 offspring. ^{[5] [14] [15]} Generally, Tuberolachnus salignus is most active in the warmer months, with populations peaking in late summer and early autumn. ^{[6] [3]} In colder moths, the overall fecundity decreases but colonies are still active. ^{[6] [3]} In the spring, colonies are usually not visible. ^{[5] [6] [3]} During this time, it is thought Tuberolachnus salignus retreats behind the willow tree bark or into the ground to escape the harsh climate. ^{[5] [6] [3]}

Tuberolachnus salignus reproduces via parthenogenesis. ^{[5] [7] [8] [9]} Resulting in a population consisting of all females. ^{[5] [7] [8] [9]} All offspring are identical genetic clones of their mothers. ^{[5] [7] [8] [9]} In turn, there are no mating behaviors that occupy these aphids as they do not participate in sexual reproduction. ^{[5] [7] [8] [9]}

Diet and Foraging

Tuberolachnus salignus is known to feed primarily on willow and Salix tree species such as the white willow ( Salix alba), the common sallow ( Salix cinerea') ^[16] and the Indian willow ( Salix tetrasperma). ^[17] It has also been recorded on Quince ( Cydonia oblonga), ^[18] Apple ( Malus spp.) ^[18] and Poplar ( Populus spp.) ^[18]

Like all Hemiptera, Tuberolachnus salignus uses modified mouth parts to pierce the phloem of plants and feed on its sap. ^{[4] [5]} As the name suggests, giant willow aphids prefer willows of the Salix species but have been known to feed on poplar, apple, and pear species in New Zealand. ^{[4] [5]} Tuberolachnus salignus prefers to feed during the day and during the warmer hours. ^[5] When feeding, the aphid will climb to the tip of the willow where the shoot begins. ^{[4] [5]} The flesh of the plant is softer here, allowing the aphid to inject its stylets into the plant’s phloem. ^{[4] [5]}

Tuberolachnus salignus uses two stylets to extract the sap flowing within the phloem. ^{[4] [5]} One is used to inject saliva into the phloem, while the other is used to ingest the sap and saliva mixture. ^{[4] [5]} Willow sap is an extremely nutritious food source, as it provides critical sugars and water to various parts of the willow tree. ^{[4] [5]} As the aphid feeds, it is ingesting an incredible amount of these sugars and water, so much so that it needs to excrete them to avoid overfeeding. ^{[4] [5] [7] [9]} Towards the posterior of the aphid are two cones called siphunculi, which are used to excrete the excess sap. ^{[4] [5]} The sap that is released is known as honeydew and attracts various other insect species such as honeybees, wasps, and ants. ^{[4] [5] [7] [9]} These other species will feed on the honeydew that is excreted from Tuberolachnus salignus. ^{[4] [5] [7] [9]}

Predators, Prey, and Parasties

Due to the almost exclusive feeding of willow sap by Tuberolachnus salignus, the honeydew that is produced as a result attracts numerous types of insects. ^{[4] [5] [7] [9]} Wasps, bees, ants, flies and coccinellids have all been observed to feed on Tuberolachnus salignus honeydew. ^{[4] [5] [7] [9]} These insects feed only on the honeydew, not the actual aphid. ^{[4] [5] [7] [9]} In New Zealand, Pauesia nigrovaria was introduced as a biocontrol parasitoid of Tuberolachnus salignus in 2020. ^[19] Elsewhere, like North America and parts of Europe, parasitic wasps, lady beetles, hover fly maggots, lace wings are frequent predators of Tuberolachnus salignus. ^[5]

Cultural Uses

Tuberolachnus salignus has only recently come into the limelight in New Zealand. ^{[9] [11]} Upon arriving here in 2013, cause for alarm was immediately raised. ^{[9] [11]} As previously noted, these giant aphids feed almost exclusively on willow species. ^{[9] [11]} As Tuberolachnus salignus populations begin to rapidly grow on the willow tree, many negative impacts start to become apparat. ^{[9] [11]}

Willow trees are an extremely important group of plants in New Zealand. ^{[7] [3]} While they are not native, they are used in many facets of society in order to increase land stabilization and reduce soil erosion. ^{[7] [3]} Historically, willows have been planted along riverbanks and on particularly sloping land to help keep the soil beneath from eroding away. ^{[7] [3]} Furthermore, willows play an important roll in honey production throughout New Zealand. ^{[7] [3] [20]} The flowers of willows are rich with pollen and nectar and attract honeybees. ^{[7] [20]} In other parts of the world, such as the U.K., large amounts of willows are cultivated for biomass production. ^[20] With the presence of large Tuberolachnus salignus colonies on willow trees, many of the contributions listed above are severely hindered. ^{[7] [3] [20]}

References

Further reading

• Collins, C. Matilda; Leather, Simon R. (2001). "Effect of temperature on fecundity and development of the Giant Willow Aphid, Tuberolachnus salignus (Sternorrhyncha: Aphididae)". European Journal of Entomology. 98 (2): 177–182. doi: 10.14411/eje.2001.033.
• Collins, C Matilda; Rosado, Rui G; Leather, Simon R (April 2001). "The impact of the aphids Tuberolachnus salignus and Pterocomma salicis on willow trees". Annals of Applied Biology. 138 (2): 133–140. doi: 10.1111/j.1744-7348.2001.tb00095.x.
• Mittler, T. E. (September 1957). "Studies on the Feeding and Nutrition of Tuberolachnus Salignus (Gmelin) (Homoptera, Aphididae)". Journal of Experimental Biology. 34 (3): 334–341. doi: 10.1242/jeb.34.3.334.