Bugged but Unbroken: Obstinate Aphids

 

I work as an annual flower vendor at a number of Home Depots near my home in Southern Ontario. A number of weeks ago as I began my day, I received a disconcerting phone call. I was told that a recent shipment of pepper plants, already nicely displayed at my stores, were infested with aphids (“Green Peach” aphids to be exact!). A sinking feeling of dread accompanied that phone call – I know from experience how people react to being told that their plants have an aphid infestation! This troublesome family of insects were designed by God to have incredible reproductive and developmental success, as well as ingenious defense mechanisms that make them both interesting to study, and difficult to eradicate. 

Fig. 1 The aphids on my pepper plants!

    These tiny green, black or white insects of the order Hemiptera and family Aphididae, are like tiny plant vampires that slowly suck the life juice (phloem) out of plant leaves and stems (Mahr, 2025). This leads to decreased plant resilience, making the plants more susceptible to disease and death. Aphids have the ability to act as vectors, transmitting viruses from unhealthy to healthy plants. These traits common to this type of insect make their presence a nightmare for gardeners. 

    Aphids use the phloem from plants to fulfill some of their nutritional needs, however, since this sap is mostly sugar, aphids must consume excessive amounts of sap to gain enough nitrogen in their diet (Mahr, 2025). Therefore, much of the excess sugar they consume is excreted as waste in the form of honeydew. 

Fig. 2 An aphid producing honeydew. 

    The presence of this sweet, sugary substance on plants becomes the perfect breeding ground for the development of a fungus called “Sooty Mold” (Mahr, 2025). Sooty Mold thrives on honeydew, and begins to grow on the surfaces of the plant that are coated in this sweet substance, eventually blocking the plants ability to absorb sunlight. Without sunlight, the plant is unable to perform photosynthesis and can eventually die (Frank et al., 2019). However, the trail of honeydew that aphids leave behind can also be used to attract other creatures (or pests!) that assist aphid survival. If the honeydew attracts ants, the myrmecophilous (ant-loving) aphids often form a mutualistic relationship with the ants. In turn for the sweet nutrition of the honeydew, ants defend aphids from their natural enemies, and herd them from plant to plant so that the aphids can continue to produce honeydew (Vela et al., 2021). 

Fig. 3 Ants herding aphids to another location. 

    Aphids certainly need the protection that the ants provide, as they have many natural predators. These include birds as well as other insects, such as ladybugs and Braconid Wasps (Miller, n.d.). However, even without the defense provided by ants, aphids have many ways in which they ensure their own survival, and the growth of their populations. The masters of excreting sticky substances, species of these tiny insects (such as Eriosoma lanigerum, or the Wooly Apple Aphid), often use their cornicles to pump out a gummy wax that coats their body and acts as “glue” on the mouths of predators – thereby inhibiting their ability to consume the aphids. 

Fig. 4 Aphid mechanism for producing defensive cornicle wax. 

    Some aphids, such as the Myzus persicae (or Green Peach Aphids) also use their cornicles to emit pheromones which are perceived by other aphids using special sensory organs on the antenna called rhinaria (de Vos et al., 2010). Alarm pheromones can be detected, and alert aphids to incoming danger (Michaud, 2022). 

    Other species of aphids show their resilience by employing other ingenious defense mechanisms. Aphis nerii (Oleander Aphids) and Uroleucon hypochoeridis (Large Cat's Ear Aphids ) greet their predators with forceful kicking of their hind legs (Hartbauer, 2010). The twitching of two microscopic legs may not seem like they would cause a lot of damage, but to a ladybug, this could feel like energetic kickboxing (Hartbauer, 2010). Other subfamilies of aphids, such as the Hormaphididae subfamily, make protective galls (which they use for feeding, reproduction and growth). To ensure the survival of the aphids within the galls, this subfamily employs female aphid nymphs that never fully mature, to protect the galls (Foster & Rhoden, 2002). If a predator attacks, these aphid nymphs, with their strong arms and legs, sacrifice themselves to detain intruders from destroying the gall and consuming the aphids residing inside (Baker, 2013). 

    In addition to these many defense mechanisms, the growth and continuation of aphid populations also relies on their quick reproductive abilities. Female aphids can reproduce parthenogenetically, and give birth to live young (Ogawa & Miura, 2014). If their population is threatened, females can produce male aphids which allows for sexual reproduction and more genetic diversity, which increases chances for adaptability and survival (Mahr, 2025). If aphids need to migrate to a new host plant, non-winged females can produce winged aphids, and these newly winged aphids can be carried by the wind to new environments (Ogawa & Miura, 2014). 

Fig. 5 The genetics of how aphids produce winged offspring from a wingless parent.

    The ability of aphids to practice viviparity also enables these insects to develop quickly, and rapidly reach maturation. Because of their reproductive success, and their many defense mechanisms, exterminating aphids is quite the challenge. Thankfully, many different techniques and pesticides exist to treat infestations of these insects. The challenge however, is to find natural techniques to deal with these invasive insects – especially those that are able to defend themselves against their natural predators (Mahr, 2025).

References

Baker, J. (2013). Witchhazel leaf gall aphid. NC State Extension Publications. https://content.ces.ncsu.edu/witchhazel-leaf-gall-aphid#:~:text=In%20the%20next%20spring%2C%20the,the%20aphids%20grow%20and%20reproduce.

de Vos, M., Cheng, W. Y., Summers, H. E., Raguso, R. A., & Jander, G. (2010). Alarm pheromone habituation in Myzus persicae has fitness consequences and causes extensive gene expression changespression changes. Proceedings of the National Academy of Sciences, 107(33), 14673–14678. https://doi.org/10.1073/pnas.1001539107

Foster, W. A., & Rhoden, P. K. (2002). Soldiers effectively defend aphid colonies against predators in the field. Animal Behaviour, 55(3), 761–765. https://doi.org/10.1006/anbe.1997.0664

Frank, S., Bambara, S., Jones, R. K., & Baker, J. (2019). Sooty molds: NC State Extension Publications. Sooty Molds | NC State Extension Publications. https://content.ces.ncsu.edu/sooty-molds

Hartbauer, M. (2010). Collective defense of Aphis Nerii and Uroleucon hypochoeridis (homoptera, Aphididae) against natural enemies. PLOS One, 5(4). https://doi.org/10.1371/journal.pone.0010417

Mahr, S. (2025). Aphids, in-depth. Wisconsin Horticulture. https://hort.extension.wisc.edu/articles/aphids-2/#:~:text=When%20these%20hatch%20in%20the,the%20adults%20but%20are%20smaller.

Michaud, J. P. (2022). The ecological significance of aphid cornicles and their secretions. Annual Review of Entomology, 67(1), 65–81. https://doi.org/10.1146/annurev-ento-033021-094437

Miller, E. (n.d.). Managing and controlling aphids. University of Washington. https://depts.washington.edu/hortlib/pal/managing-and-controlling-aphids/#:~:text=Aphids%20have%20many%20natural%20enemies,aphids%20when%20conditions%20are%20right.

Ogawa, K., & Miura, T. (2014). Aphid polyphenisms: Trans-generational developmental regulation through viviparity. Frontiers in Physiology, 5. https://doi.org/10.3389/fphys.2014.00001

Vela, J., Montiel, E. E., Mora, P., Lorite, P., & Palomeque, T. (2021). Aphids and ants, mutualistic species, share a mariner element with an unusual location on aphid chromosomes. Genes, 12(12), 1966. https://doi.org/10.3390/genes12121966