Ahhh Spring. It’s that time of year when the greenhouse
gardener’s thoughts and dreams turn to … aphids.
Specifically, to how to make
them go away.
Based on my research, the spring aphids that are
congregating on my radish leaves and broccoli seedlings are probably green
peach aphids, even though they are not green. Whitney Cranshaw states in Garden Insects of North America that
this aphid is “one of the most commonly damaging aphids of greenhouse crops.” I
have questions about Cranshaw’s syntax (What do you think? Should “commonly” modify “damaging,” or should “common”
modify “damaging insects”) but none at all regarding his entomological prowess.
I will accept that my brown aphids are really green. If Whitney C. says so.
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| Note the ghosts of aphids past. |
I got very excited seeing ghostly, leggy forms scattered
among the plump marauders. The lacewing eggs I scattered two months ago must be
hatching (!), I thought, and tiny lacewings are sucking the sweet life out of
the plant suckers. But no. Between birth and adulthood, which takes all of 8
days, the aphids molt four times. The white forms are the skins they’ve shed
along the way.
The thing about aphids is, if you miss them early on, your
crop is doomed, your ship has sunk. Embryos start forming in their grandmother,
making it possible for an adult female to give birth to as many as a dozen live
nymphs a day. (1) Count it up: that’s
84 a week. From a single aphid. Meanwhile all those babies are shedding and
maturing and, by the end of the week, having live babies of their own. I will
leave it to the mathematician to calculate a sum total after a month of
unchecked reproduction, should you miss that initial batch of a dozen or so
aphids.
Trust me: It is a big, big number.
And this is why, when the box of 4500 Asian ladybugs (Hippodamia convergens) arrived at my
door late Friday afternoon, I rushed right over to the greenhouses to release
them. No, these are not the same Asian ladybugs that enter your house in winter
and crawl around on your windows and walls, in case you were wondering. Those are
Harmonia axyridis. They look very similar, but they’re not the same.
According to Whitney C.
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| Prey is multitudinous. And conveniently slow! |
I watered the
greenhouses well, and tenderly situated my new allies in protected places. I
nestled them among the carrot foliage and gave them shelter beneath broad
leaves of chard, I gently shook a few out of the bag to populate the weeds
along the greenhouse edges and rested others in the dried remains of last
fall’s nasturtium vines. The next day, Easter Sunday, was cloudy and
rainy—perfect for rest and recuperation after a 2-day journey from Arizona,
cramped 281 (give or take a few) per cubic inch.
On Monday (I am making an assumption here) they reveled in the bounty. What ladybug would not be thrilled to come
upon a batch of tender, newly hatched, sap-sucking insects. What ladybug would not get right to
work consuming their quota of about 22 aphids a day. After all—and this is a
fact—unless she eats aphids, she will not lay sticky bunches of yellow eggs
that, in a little over a month, will become hungry larvae. A built-in mechanism
causes the developing eggs to be reabsorbed into her body unless there is a
proper food supply about.(1) How civilized.
But, assuming that aphids abound
(which they do, in an April greenhouse that has been kept above freezing all
winter), what happens next is a thrilling thought to one who wakes up to dreams
of hoards of hungry sucking beasts destroying crops of baby bok choy: When the
ladybug larva emerges from the yellow egg and encounters an aphid, it bites a
hole in the body and sucks out the contents. Then it pumps the liquid back
into the body and sucks it out several times to effectively mix the innards of
their victim with digestive juices.(2) The mere thought makes my sleep more restful.
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| Grey cabbage aphids like the cool weather of fall. |
As a gardener, it’s difficult not to think in terms of the
good, the bad, and the ugly. It’s easy to presume that one is winning the game,
that ladybugs will solve the problem (and my, aren’t we clever). The truth,
however, is far more intricate. The grey cabbage aphids of fall are still
hanging about in small clumps here and there, waiting. Waiting for the tides to
turn, waiting for the savoy cabbage to make an appearance, so that they can
take instant advantage of its protective crinkles and give almost immediate
birth to practically countless young who will keep the cycle alive. I will
refrain from planting brassicas in certain problem areas in spring. I will put
distance between the consumer and its favorite food. It may work. It may not.
As savvy as I think I am with my ladybugs and my strategic
planting plan, my means are primitive in comparison to those of the tiny aphid,
whose hollow stylus, thinner than a human hair, both pierces and sucks; who
comes equipped with exudation tubes that drip sweet honeydew so rich in sugars that
ants will protect the exuders so as to keep the tap flowing; whose generation
time can be a mere 10 days, which allows them to adapt and evolve as circumstances
change. Who does not give birth unless there is an adequate food supply. Who
(unlike us, who must eat carrots and yams) can produce its own carotenoids! (3)
"Treating your adversary with respect is giving him an advantage to which he is not entitled," said poet, writer, and moralist Samuel Johnson. I disagree. The mighty aphid is deserving of respect.
Awe, even.
(1) http://online.sfsu.edu/bholzman/courses/Fall99Projects/ladybug.htm
(2) http://faculty.ucr.edu/~legneref/identify/ladybird.htm
(3) http://www.nsf.gov/news/news_summ.jsp?cntn_id=116842
