Master Gardeners In the Garden
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Feed the pollinators in your garden, not the aphids
By Julie Banken
WSU Chelan/Douglas County Master Gardener
place holder Julie Banken – WSU Master Gardener intern – Provided photo/WSU Master Gardeners Aphids are phloem feeders, which means they use their straw-like mouthparts to feed on plant sap. Like poking a hole in a pressurized garden hose, all an aphid has to do is plug its stylet into the right spot and let the plant’s juices flow.
The best place for an aphid to find food, then, is where its host plant is actively growing. New stems, leaves and flower buds are made of soft tissue that is easy to pierce and especially rich in sugars and amino acids.
Plants that slow their growth in the summertime generally are less appealing to aphids. When temperatures rise, their leaves get tough and their flowers go to seed. Plants that keep growing throughout the summer, in contrast, are excellent aphid hosts, and the tender leaves of flowering petunias are among their favorites.
It’s important to appreciate how quickly aphids can reproduce. Winged females are usually the first to colonize a new host plant. Once they pierce the soft leaves and taste the sweet phloem of a well-watered petunia, they settle in. They don’t mate or lay eggs; instead, these females can give birth to up to 100 new aphids by cloning themselves. Their offspring are wingless, so they stay and feed on the plant where they are born. It only takes about a week before a newborn aphid gives birth to her own flightless offspring. That’s when the math quickly gets complicated. It’s simplest to say that on a petunia plant, aphid populations can balloon rapidly.
It might seem like a good idea to give your petunias an extra boost of fertilizer if you notice that aphids have discovered them. Surprisingly, adding nitrogen can backfire. Fertilizer stimulates the new plant growth that aphids love and only makes the plants more appealing. Those extra nutrients will just end up feeding the aphids, too.
Petunia plants aren’t defenseless. The leaves and stems of many cultivars have microscopic hairs called trichomes that give off sticky secretions, making them feel tacky to the touch. Being sticky deters small, chewing insects, making it harder for them to feed or lay eggs. Unfortunately for petunias, aphids don’t chew. Their long, skinny legs keep their bodies above the sticky trichomes on petunia leaves. Once they plug into a leaf, they rarely even need to move. They just remain in place, feeding and reproducing.
The more aphids that feed on a plant, the more sap it loses. Imagine a garden hose with thousands of holes; its pressure would drop significantly. In a plant, leaves turn yellow, and growth slows as aphids siphon away sugars and amino acids. Heavily infested petunias become a sticky mess, covered in honeydew and dotted with tiny white aphid exoskeletons.
If aphids are multiplying on your petunias, resist the temptation to reach for harsh chemicals. Protecting them can come at the cost of harming beneficial insects. Broad-spectrum topical sprays can knock aphid populations back, but they will also kill predators that naturally help keep pest populations down.
In addition, nursery plants that have been pre-treated with systemic insecticides can inadvertently expose unsuspecting pollinators and their offspring. Systemic insecticides last for weeks or even months, spreading internally to all parts of the plant, including its nectar and pollen.
When it comes to aphids, work with nature rather than against it. Instead of fighting off aphids with chemicals, avoid feeding them their favorite foods. Petunias aren’t the only beautiful summer bloomers; there are many other plants like million bells, verbena, and geraniums that add color to baskets and borders, but with tougher foliage and different plant chemistry; they aren’t as appetizing to aphids.
Even better, consider going native. Native plants in Eastern Washington are not aphid-proof, but they are less prone to severe outbreaks than ornamental annuals. Many are adapted to dry summers and nutrient-poor soils, so tend to grow slowly and have tough tissues. Their flowers may not be as long-lasting as petunias, but they are not just for decoration. They serve a higher purpose, sustaining the pollinators and other beneficial insects while helping create a resilient ecosystem that naturally keeps pest populations in check.
A WSU Chelan and Douglas County Master Gardener column appears weekly in The Wenatchee World. To learn more about your local Master Gardener program, visit bit.ly/wsucdmgprogram or call (509) 667-6540.
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