It’s August, you’re walking to your car parked on a side street. In the cool shade of the trees you notice something sticky coating the sidewalk. It’s summer, so you think nothing of it – it’s probably just a child’s spilt apple juice dried upon the sidewalk. But you get to your car and…..
…your car is thick with the same sticky mess, as if it rained honey while you were away. You get in, but the door handle is sticky and you’re using most of your windshield fluid in hopes of washing it off just to try and get home. What is the sticky substance? …
That’s right. Aphids feed on plants by putting their piercing mouthpiece into plant tissue and drinking the phloem (or sap). They excrete honeydew. By summer, some boulevard trees have such large aphid populations that you can feel the honeydew raining down as you walk below them. Honeydew is water soluble, and more of a nuisance than anything, but can become a problem on plants where it allows molds and mildews to infect other plants.
Most trees take care of the aphids themselves, as natural predator populations slowly build over the summer and eventually take over. But for trees in cities, where there are fewer predators, or trees near roads where the air is drier and hotter (thus deterring predators) control of aphids to reduce the honeydew mess is a major concern and requires intervention.
Most municipalities that have adopted a ban on chemical-use have turned to Aphidoletes aphidimyza for aphid control. None have applied this bio-control as spectacularly as Victoria, British Columbia, Canada.
Since 1937 the city of Victoria has placed flowering hanging baskets around their city each spring. All year they maintain the baskets for a burst or colour and liveliness that in 1937 and for many decades to follow made Victoria stand out as “European,” or “British.” Now, the hanging baskets may elicit memories of elsewhere, but most attribute the hanging baskets to Victoria.
For several decades all hanging baskets have been produced entirely within the City’s Parks Department located in Beacon Hill Park. This year (2021) 1250 hanging baskets were created from stock they grew on site, and all will be placed so that almost every single road in Victoria will host these beautiful baskets.
To handle the aphid populations, the city of Victoria combined their hanging basket distribution with their Aphidoletes aphidimyza (A.a.) release by placing A.a. pupa in every single basket before they are sent out. Within a few days all 1250 hanging baskets become release points of A.a. that quickly seek-out and eliminate aphid colonies – before they get to the point of creating the honeydew nuisance.
The benefits are huge:
city wide distribution
reduction in chemical use
promotion for other natural aphid predators to establish
Private residence get aphid control by proximately.
Public spaces like parks and courtyards sustain predator populations
Public sees the city as being proactive, responsible and sustainable.
The principle is simple. Aphidoletes emerge as adults from the hanging baskets. The adults only eat honeydew (another huge benefit for using A.a.). The find aphid populations and lay their eggs on or near the aphids. Each female can lay hundreds of eggs. The hanging baskets may have aphids, but the Boulevard trees (at the time of release) certainly do. Larva emerge and each kill hundreds of aphids and consume some of them before finding a place to pupate (usually in soil.) Two weeks later, approximately 20x the original release rate fly up, mate and start the cycle over again. An adult A.a. can search out and find aphids over a hectare and so they spread out quickly and begin their coverage.
Victoria is not alone in large public releases of Aphidoletes, but they are among the most successful. Other cities struggle with citizens using harsh chemicals on lawns which interfere and kill aphid predators in the trees. (Some need to pupate in the ground.) Some cities in colder climates struggle in waiting long enough for outdoor temperatures to be optimal for A.a. releases, while getting A.a. out soon enough to match the aphids quick growth. If they wait too long, more A.a. are needed and the cost goes up. Apply A.a. too early and frost may set their population back. Luckily this is only the case in some northern, mid continent cities. For everyone else, A.a. releases can occur as early as mid April – long before most aphid populations establish.
For everyone else, (like a home gardener) A.a. can be released between late April and September for cycling and anytime above freezing for just a single generation. In darker months of the year the Aphidoletes larva (after eating aphids) will pupate until the soil warms in the spring. Aphidoletes is the industry standard bio-control agent for controlling most species of Aphids.
So, unless you live in a city like Victoria, where you benefit from the public release of Aphidoletes, you may want to inoculate your own yard. Lucky a typical garden only needs a single release of 250 A.a. which might set you back $20-$30. A small price to pay for no honeydew, or to protect some of your cherished plants.
However, you could also encourage your municipality to do the same. While few provide hanging baskets, most municipalities have parks or areas for bedding flowers, that can be used for the same purpose. Even if you don’t benefit from Aphidoletes finding their way from public spaces into your garden, at lease you’ll know your city is using less chemicals and engaging in sustainable practices. So, please. Let them know!
Thank you to the City of Victoria and the Parks Crews for letting me tag along and take pictures.
For more, visit Victoria.ca/hangingbaskets
For more on Aphidoletes aphidimyza, click here.