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From the Hort Desk – Host an Aphid Watch Party This Summer and Look For Biodiversity

By Lisa Mason, CSU Extension Horticulture Agent

Yes, you read that right! Get excited about watching aphids on your plants this summer just as you get excited about your favorite TV show or a new movie coming out. You’re probably asking, “Why would I watch aphids on my plants? Aren’t they a nuisance pest?”

The definition of a nuisance pest depends on who is asking. For instance, if you are a hungry lady beetle, aphids are a feast waiting to be had.

Except for a few species, most aphids cause very little plant damage. Aphids are host-specific, meaning that each of Colorado’s 350 aphid species will only feed on certain plants. 

Aphids are also one of the insects that excrete honeydew, which is a sweet, sticky substance that coats the plants and anything underneath the plant.  

So, why would you watch aphids on your plants? Aphids attract a wide variety of beneficial insects to our gardens. An aphid infestation can be a hotspot to observe biodiversity in your garden or landscape. For instance, while I was writing this article, I found aphids in my backyard on a sand cherry. So far, I’ve seen lacewing eggs, lady beetle pupa, a katydid nymph, syrphid fly smears, and other insects feeding on the honeydew.

Aphids (Aphis nerii) on a milkweed plant. Photo: Lisa Mason

Here are some fun observations you may witness while you are watching aphids: 

Predators Hunting Aphids 

Lady beetles (Family: Coccinellidae), also known as lady bugs, are a common insect you’ll find preying on aphids and other soft-bodied arthropods. We have approximately 80 different lady beetle species in Colorado. Lady beetle larvae have a completely different appearance than the adult beetles because they complete a full metamorphosis (egg, larva, pupa, and adult). The larvae look similar to little dragons or alligators, and they are ferocious predators. Their goal is to eat as much as possible! Convergent lady beetle larvae can devour up to 50 aphids a day, often consuming their weight in aphids (Hoffman and Frodsham, 1993). Other species such as the seven-spotted lady beetle may feed on higher numbers of aphids.

Lady beetle larva. Photo: Melissa Schreiner

Lady beetles are a natural control option for aphids found in their environment. They are available for purchase at garden centers, but keep in mind that lady beetles can fly away. Since they are so mobile, lady beetles for purchase may not be too effective. However, purchasing and releasing lady beetles can be a great activity for children to help inspire an appreciation for insects and their role in our environment. The lady beetles for purchase are the convergent lady beetle, a native species. 

Female lady beetles often lay eggs near an aphid infestation, so look for groups of dark yellow or orange-colored eggs that are oval-shaped and approximately 1 mm long. 

A lady beetle larva preying on an aphid. Photo: Melissa Schreiner

Lacewings (Family: Chrysopidae) are another common predator seen feeding on aphids. Just like lady beetles, the larvae of lacewings are ferocious predators. They earn the nickname “aphid lion” for a reason. In addition to eating up to 200 aphids per week, they can also feed on caterpillars by capturing them with their pair of hooked jaws. 

Both green and brown lacewings can be found in Colorado. The adults have long, skinny bodies with membranous wings that extend over the body forming a triangular or tent-shape. 

Lacewings are also available for purchase as a biocontrol for the home garden. They can be a more effective biocontrol option because they are available as eggs. When they hatch, the larvae will stay in the same place to feed until they reach adulthood. 

Syrphid flies are another insect that feed on aphids as larvae, but they are quite elusive. Larvae can feed on 100-400 aphids. They are nondescript, grub-like larvae that vary in color. They leave behind excrement in their path that looks like a black, shiny streak known as a “syrphid smear.” 

The adult form of syrphid flies is commonly seen visiting flowers with nectar. They often look like wasps and bees because they exhibit Bastian mimicry, i.e., predators may avoid these syrphid flies because they look like stinging insects, though the syrphid flies are harmless. You can differentiate them from wasps and bees by looking for their giant eyes that extend almost to the top of their heads, only one pair of wings, and short antennae.

A syrphid fly. Photo: Lisa Mason

Tiny, parasitoid wasps will prey on aphids by laying eggs inside the aphid. The wasp larvae feed on the aphid. Parasitoid wasps cannot sting people, but they are deadly to aphids. The wasps are so small that they may be hard to observe, but you might see the remnants of them when you see dead aphids on the plant.

Insects Feeding on Honeydew 

Aphid infestations also attract a wide diversity of beneficial insects that feed on the sweet honeydew. You might observe a variety of wasps, ants, flies, and beetles all feeding on honeydew. 

Ants and aphids often have a mutually beneficial relationship. Honeydew can be a major food source for ants, and the ants want to protect that food source. Since aphids are easy prey, ants will protect aphids from prey, and in some cases, protect them from fungal pathogens (Nielsen et al., 2009; Rathcke et al., 1967). 

Aphid Reproduction and Life Cycle  Most aphids are wingless in a colony. You may observe winged aphids when they need to seek out new plants. Aphids reproduce at an astounding rate because they reproduce both sexually and asexually. During the summer, you are likely to see aphids produced by females in asexual process called parthenogenesis. The female aphids give birth to aphids without fertilization. These aphids share the exact same genetics. Later in the season, the colony will produce male aphids to mate with female aphids that are able to reproduce sexually. The female will lay fertilized eggs that overwinter. Many aphid species will complete their life cycle on one host plant. Other species will overwinter on one species of host plant, and after hatching the spring, they will fly to another species of host plant. See Table 2 of the Aphids on Shade Trees and Ornamentals fact sheet for a list of aphids that have two host plants.

Rose aphids (Macrosiphum rosae) on a rose bush. Photo: Melissa Schreiner

Benefits of Attracting Insects in Your Landscape

Aphids and other pest insects serve an important role in the food web in our garden ecosystems. Just like predatory insects, birds can help keep the pest populations down. Most terrestrial birds rely on insects for a major component of their diet. Hummingbirds will catch up to 2,000 insects per day! They feed on aphids, mosquitos, and a variety of other insects. Consider supporting insects and birds in your landscape by planting a diversity of flowers with nectar and native plants, providing habitat spaces and water sources, and minimizing or eliminating the use of pesticides in the landscape. 

Aphid Control Options

Beneficial insects are a great control option for aphids. A few other options are available if additional control is desired. Using a hose with a strong jet of water on the plant will knock aphids down. Since they are soft-bodied insects, the impact from the water will kill many of them. Horticultural oils are effective at killing overwintering eggs. A variety of insecticide products are available, but use caution because many will kill the beneficial insect predators in your garden. Look for non-persistent contact products like insecticidal soaps that are safer for beneficial insects. As always, follow the label explicitly so the product is applied effectively and safely. For more information, check out the CSU Extension fact sheet, Aphids on Shade Trees and Ornamentals

Enjoy Your Backyard Biodiversity! 

Have fun making observations this summer. Share photos on social media! Use the hashtag #AphidWatchParty 

References 

Hoffmann, M.P. and Frodsham, A.C. (1993) Natural Enemies of Vegetable Insect Pests. Cooperative Extension, Cornell University, Ithaca, NY. 63 pp.

Marvin, D. E. (2018) Hummingbirds as Pest Management Partners? New York State Integrated Pest Management, Cornell University. https://blogs.cornell.edu/nysipm/2018/10/11/hummingbirds-as-pest-management-partners/

Nielsen, C., Agrawal, A. A., and Hajek, A. E. (2009) Ants defend aphids against lethal defense. Biology Letters 6: 205-208. https://doi.org/10.1098/rsbl.2009.0743

Rathcke, B., Hamrum, C., and Glass, A. W. (1967) Observations on the interrelationships among ants, aphids, and aphid predators. The Michigan Entomologist, 1(5), 169-173. https://scholar.valpo.edu/cgi/viewcontent.cgi?article=1056&context=tgle

Schuh, M. 2022. Lacewing. University of Minnesota Extension. https://extension.umn.edu/beneficial-insects/lacewing

Statewide Integrated Pest Management Program. (2002) Syrphids (Flower Flies, or Hover Flies). University of California Agriculture and Natural Sciences. https://www2.ipm.ucanr.edu/natural-enemies/syrphids/

Warner, G. (1993) Syrphid Flies (hover flies, flower flies). Washington State University. http://treefruit.wsu.edu/crop-protection/opm/syrphid-flies-hover-flies-flower-flies/

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