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War of the Roses: Parasitic Wasp vs Aphid

May 30, 2023

By Abbie McHardy, Student Entomologist, Royal Botanical Gardens

As the weather gets warmer, spring and summer wildlife is becoming more active at RBG. Of course there are the robins, sparrows, and other bird life that make the gardens their home, but a closer look at the plant leaves in the Hendrie Park Rose Garden will reveal a smaller, but no less complex ecosystem.

The insect world is beginning to awaken from the long, cold winter! Aphid life cycles are already underway for the season, with discarded aphid exoskeletons clinging to the undersides of the rose leaves as proof.

Abandoned aphid exoskeletons on the underside of a rose leaf
Abandoned aphid exoskeletons on the underside of a rose leaf. This is clear evidence that aphids are molting, or shedding their skin as they grow and continue their life cycle. Each individual aphid will molt approximately four times during their life.
An aphid feeding on a rose leaf.
An aphid feeding on a rose leaf.

As the Student Entomologist this summer at RBG, I’m studying the insect diversity present in the Hendrie Park Rose Garden. While walking amongst the roses in Hendrie Park, I made a sinister yet exciting discovery. The blown-up, paper-like exoskeleton of an aphid, plastered to a rose leaf, complete with a precisely-placed exit hole located on the back of the abdomen. This was the work of a parasitic wasp.

Parasitic wasps are very tiny, and make aphids their hosts. Female wasps will pierce their ovipositor through an aphid’s abdomen and lay an egg inside the aphid’s body. When the egg hatches, the wasp larva will feed on the aphid’s insides. Once the larva is ready to pupate, it will do so inside the aphid’s body, expanding and hardening the exoskeleton, “mummifying” the aphid. When the adult wasp emerges from its’ pupa, it cuts a very precise hole in the back of the mummified aphid’s abdomen and escapes to find another aphid with which to repeat the cycle.

Abandoned aphid exoskeletons on the underside of a rose leaf.
This mummified aphid is much larger and more rigid than the regular exoskeletons that are shed by aphids. This happened due to a parasitic wasp developing inside the aphid’s body. The hole in the back marks the adult wasp’s exit door.
Aphid Mummy

In 2018, the Hendrie Park Rose Garden was rejuvenated with the incorporation of additional plants alongside the roses. These ‘companion’ plants would hopefully function to increase abundances of beneficial insects in the garden. This mummified aphid is very exciting proof that this plan is working! Plants such as ornamental onions have attractive qualities to parasitic wasps and other beneficial insect life. The companion plants are functioning according to plan, and will hopefully continue to do so as the years go on at RBG.

As an emerging professional in Entomology, the rose garden has provided me with a classroom in which to further develop my skills and expertise. Spending a portion of each day in the garden, I have developed a lens through which to observe the garden from an entomological perspective. I see the flowers and plants, but I also see another layer. As I walk slowly through the garden, I am carefully observing individual rose leaves in search of anything that looks out of the ordinary. I have learned that many of the abnormalities that can be observed on and around plant life are signs of insect life.

As I finish up my first month here at RBG, I am working on compiling a phenology journal. I walk through the rose garden in search of insects in their early stages of life, indicating that their spring emergence or hatching time is among us. I am taking note of the observation date, as well as the flowers that are in bloom on and around the same day. This journal will equate spring emergences of insects with bloom times of the flowers in the garden. The insect data that I collect will be useful for the future of RBG’s integrated pest management approach (IPM) and informing the work of entomology students in years to come.

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