Slowly but surely, the season is advancing, and Spring will soon be here. The new growing season starts with such optimism! So few weeds, new seedlings to nurture, not much watering to do…then the rude awakening of the first aphid outbreak, sometimes followed by slug devastation or other problems that are visited upon gardeners year after year.
Before rushing to the local garden center for preemptive remedies – chemical or organic – let’s take a second look at the cycle of natural disasters in the backyard garden. Aphids are a good example. One year I noticed the usual profusion of aphids coating the new leaves of a Daphne odora shrub. One’s first reaction is to start stripping them off the leaves, or washing with water, soap, anything that will get the numbers down. How do aphids accomplish this ridiculous population explosion practically overnight??
It turns out that aphids have evolved a (some would say) fiendish reproductive strategy that allows them to give birth to live young without bothering with sex. They are virgin births (parthenogenetic in scientific parlance), and all the offspring are females who are born pregnant! Eventually at the end of the season when it is time to lay eggs, they do recombine genes via sexual reproduction, which keeps them genetically adaptable.
Insects are all prone to “boom and bust” population cycles, so later (or, more commonly a year or two from now) you may find very few. After hatching in the spring, the population can easily boom before the many predators that eat aphids have a chance to find them and start in on the smorgasbord. However, this does not continue for long before predators do begin to arrive, so killing aphids at this stage can short-circuit natural pest control cycles by reducing the food supply for the predators you want to attract. The inevitable conclusion is that some pests are necessary to feed the ones beneficial to our crops.
Back to my Daphne plant: as the season progressed, I noticed two things – more insects hovered about the plant, and the leaves began to mature. As the leaves toughened up, they became less palatable to the aphids – the plant is not without its own defenses. As for the hovering insects, although I didn’t recognize all of them, I saw some of my favorites: hover flies, also known as syrphid flies.
Like many insects beneficial to gardeners, the adults are pollinators, feeding on nectar and pollen. It’s the larvae that are predators of insects like aphids (aphids are the cattle of the insect world – a plentiful and docile food source for higher level consumers). Hover flies often appear to be bees, and have similar coloring in order to fool their predators, but you can tell a fly by it’s amazing helicopter-like flying abilities, having only 2 wings, and – yes – hovering near flowers where it will be looking for nectar or a place to deposit eggs. But the life stage that eats other bugs has a rather high yuck factor – it’s the larva.
The legless larvae of flies (maggots) are repulsive to many of us. So it’s hard not to want to do away with EVERY crawling thing on a plant that is obviously under attack. But if you restrain that urge and look closely you may find the predators among your pests. You may witness them eating those pests if you’re lucky. I collected some slimy-gooey looking creatures that were definitely larval something residing on the Daphne and put them in a jar with air holes and a supply of aphids, hoping they would get what they needed. After a couple of weeks of not much going on, I looked in to find the larvae were completely missing. Opening the jar, I discovered hover flies on the inside of the lid. Voila! Now I know what hoverfly larvae look like.
I invite all gardeners to try collecting and watching larvae transform (if you have kids, it’s great with ladybug larvae – just make sure they have enough leaves with aphids because they eat a lot and it can be quite a job to collect them). The event of metamorphosis has a completely time-bending, otherworldly quality – one day you have a larva or pupa, and suddenly it’s gone – yet the organism is still there. I expect this process to be accompanied by orchestras and choirs, yet it goes on silently every day thousands of times.
The Neighborhood Naturalist has produced in the 2012 Spring issue, an excellent in-depth article on hoverflies, with stunning close-up photos. Read it here – digital subscriptions are FREE.
So this year, try these successful strategies for encouraging your beneficial allies in the garden:
- Take a deep breath and count to ten when you see insects
- Figure out what’s eating what (Master Gardener clinics are good places to take your samples, with the plant parts you find them on)
- Protect the predators by not spraying poisons
- Predators include birds. Give birds and insects a water and food supply (many beneficial insects especially like flat-topped flower clusters like dill, cilantro, parsley and yarrow, and all adults need nectar and pollen). Planting a hedgerow with shrubs, perennials and trees gives both insects and birds a nice habitat to forage in. Be sure to leave mulch on the ground for predatory beetles and spiders to hide in.
- If you need to salvage your plants from the pests, leave some behind for the predators to eat. You can trim off affected parts, or even have a sacrificial area where you don’t control any pests.
Thanks to Bruce G. Marcot for the hover fly photo.
The Aphid Life Cycle. The Backyard Nature Website http://www.backyardnature.net/aphid_lc.htm.
Images of hover flies and larvae http://www.google.com/images?q=hover+flies&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&source=univ&sa=X&ei=Aq5hTYXpFoWqsAOG9ITtBg&ved=0CEsQsAQ&biw=972&bih=619
Basic life history of hover flies http://aggie-horticulture.tamu.edu/galveston/beneficials/beneficial-28_hover_or_syrphid_flies.htm
Ladybug larvae images http://www.google.com/images?oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&q=ladybug+larvae&um=1&ie=UTF-8&source=univ&sa=X&ei=v7thTfCbPJK8sQOozaHACA&ved=0CCYQsAQ&biw=972&bih=619