Beautiful Native Plants


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Monday, February 6, 2017

Biocontrols. Fighting Fire with Fire

Mile-a-minute choking trees
© Beatriz Moisset
 What to do when an introduced species, be it a plant or insect, gets so out of control that eradication seems impossible? This is a serious consideration. We see it happens again and again. Many species are so entrenched that all efforts seem futile, from the most ecologically benign, such as physically removing the offenders, to the least desirable ones, the use of pesticides.

Part of the reason why some introduced organisms become invasive is that they left behind the enemies that kept them in check in their native lands. Once free from such restraints they can become enormously successful in their adoptive land. When other methods of control fail conservationists have been resorting to what I would call fighting fire with fire: bringing another introduced species to control the one that has become a pest. These are the so called biological controls or biocontrols. This method is a partial admission of defeat. It allows the invasive species to remain but minimizes its impact. The National Wildlife Federation says it best:
In vast natural areas such as the Everglades, where spraying, snipping or bulldozing exotic weeds is out of the question, a well-tested, well-behaved bug is the conservationist’s best weed whacker.
Vedalia beetle feeding on cottony cushion scale
© Matt Bertone
In the 1880s California citrus growers were desperate because an introduced pest, the cottony cushion scale (Icerya purchasi) was destroying their crops and nothing seemed to stop it, even burning infected trees. Scales are similar to aphids in that they feed on the sap of plants. This one, as the name indicates is coated with a white cotton-like material.

In desperation, they imported a lady beetle from Australia, the Vedalia beetle (Rodolia cardinalis). It adapted well to its new environment and went right to work bringing the pest under control and saving the citrus industry. This was the beginning of biocontrol introductions.

However, the method may backfire with disastrous consequences. At first, it was thought a good idea to bring biological controls with wide tastes. It turned out that some went after desirable bugs, not just the undesirable ones. One of the worst examples was a parasitic tachinid fly (Compsilura concinnata) brought in to control the gypsy moth. Unfortunately, it started feeding on many native moths including the spectacular Cecropia moth. Populations of some of these moths have dropped down enough to cause serious concern.
Cecropia Moth (Hyalophora cecropia)
© Marvin Smith. Wikicommons
It took some time to learn this hard lesson. Many biocontrols have created more problems than benefits. I mentioned the Chinese praying mantis and the Asian lady beetle in previous posts.

Nowadays the only biocontrols allowed are those with extremely narrow feeding habits, ideally only the target species. The U.S. Department of Agriculture approves biocontrols only after they have been subjected to thorough testing under tight quarantine conditions.

I will illustrate this with two examples that I have had the opportunity to observe: one controls an insect pest, the other a weed.

Japanese beetle with egg of parasitic fly
© Beatriz Moisset
The Japanese beetle has been advancing inexorably ever since it arrived in New Jersey hiding in some horticultural stock. Recently, a parasitic fly has been introduced to combat it. The winsome fly (Istocheta aldrichi) lays one or a few eggs on the back of the beetle. When the larvae hatch they penetrate the hapless insect and proceed to eat its insides. If you see a Japanese beetle with a white dot on its back, don’t kill it. Let the parasite complete its life cycle and live to infest other beetles.

Mile-a-minute weevils on mile-a-minute vine
© Beatriz Moisset
Mile-a-minute is becoming a terrible pest climbing up trees and choking them. Recently its biocontrol, a weevil, has been approved for distribution. The beetle is not much bigger than the tip of a ball point pen. It feeds on tender leaves. If you see holes in full size leaves, don’t look for the weevil there; instead follow the tendril to its end. This is where these voracious beetles hide. This introduction is so recent that we probably haven’t seen its full effect.

© 2015, Beatriz Moisset. First published in Native Plants and Wildlife Gardens 2/7/15