Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
2/17/03
Contacts:
Donald Lewis, Entomology, (515) 294-1101, drlewis@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning February 21, 2003
Where Have All the Insects Gone?
By Donald Lewis
Extension Entomologist
Iowa State University
No doubt about it. Winter can be a tough time to live in Iowa. Animals and plants that live and prosper here do so by having some "trick" to escape the periods of inhospitable weather, and winter can certainly be inhospitable!
Insects are cold-blooded animals. That means, like snakes, frogs and turtles, the body temperature of an insect varies with the environmental temperature. When it is hot, insects are hot. When it is cold, insects are cold. And when it is very cold, insects are inactive (dormant).
Dormancy is an inactive state when insects stop moving, stop eating and stop growing until better weather returns. Generally, insects are dormant at temperatures lower than 45 degrees F.
Dormancy allows insects to survive short periods of bad weather but prolonged dormancy can be fatal unless the insect was prepared to make it through the long run.
A recurring period of inactivity such as winter dormancy is called diapause, a word that could be translated from its Greek roots to literally mean period of cessation. Diapause causes several changes in metabolism and behavior, the most obvious of which is reduced activity. Diapause can occur in any life stage of insects, though, for each different species, diapause occurs at the same stage, every time.
Dormancy seems like a pretty slick way to avoid the adversity of winter, doesn't it? But how can an insect, with a simple nervous system and no known abilities at clairvoyance know when winter is just around the corner?
It turns out most insects initiate diapause in response to a specific environmental cue. Although some insects go dormant because of changes in moisture or temperature, winter diapause is most often triggered by the shortening day length of late summer, which is after all, the only completely reliable clue that winter is approaching.
Insects in winter diapause are further protected by antifreeze-like compounds that protect against ice formation in their bodies. Insects in diapause need a little energy that they get by feeding off their stored body fat.
Where are the butterflies?
Iowa butterflies are survivors. They seem to be better equipped for this
frigid time of year than other insect species. They survive in different
ways and in different stages.
Our best-known butterfly, the monarch, doesn't stay in Iowa at all. The adults we saw last fall were all headed to Mexico where they will rest until next spring. Any that stayed in Iowa rather than going south with the others, froze to death long before now. Fortunately, plenty made it to Mexico to return and perpetuate the species by laying eggs on milkweeds in Mexico and Texas.
The swallowtails such as the black swallowtail that fed on your dill plants as a caterpillar spend the winter right in the thick of it. The butterfly pupa, called a chrysalis, was strapped to standing host plant debris and waits there until spring before finishing the transformation into an adult.
The cabbage white butterflies that come from the imported cabbageworm caterpillars that were eating your broccoli and cabbage foliage last summer are also waiting for spring as chrysalides. They are suspended from any plant debris or nearby objects. These butterflies are early birds and are one of the first butterflies to emerge as adults in the spring (probably to get a head start of scooping out your garden for cabbage plants!).
Only a few butterflies are able to make it through the Iowa winter as an adult. The brightly colored painted lady and red admiral may hibernate as adults or chrysalides. The same applies to the orange-colored alfalfa butterfly. The truly hardy butterfly is the mourningcloak. This common, but rarely noticed butterfly is up to 3 inches across. The wings are dark purple with a wide creamy yellow border at the outer edge.
Do butterflies need help?
Butterfly boxes are also known as butterfly roosts and hibernation houses.
Advertisements claim these bird house-sized structures encourage butterflies
to stay in your garden by providing necessary shelter. It turns out the
butterflies couldn't care less. Occupants of butterfly houses are almost
always spiders, wasps or mice rather than butterflies. The few butterflies
that do remain for the winter seem to find sufficient natural hiding places
under dead leaves, twigs and woodpiles.
It is tempting to dismiss butterfly houses as worthless, but that's not quite true. They can be an attractive and artistic addition to the garden and they also announce to visitors (that is, people) your intentions to have a butterfly-friendly place. Let's keep it that way.
-30-
ml: isugarden
|
Editors: A color photo, suitable for publication, is available at right. Click on the thumbnail photo to go to the fullsized photo. The picture's fullsize photo is 496K. Caption: Giant swallowtails. |
 |
Extension programs are available to all without regard to race, color, national origin, religion, sex, age, or disability.