Miconia calvescens DC. (Melastomataceae) is a striking plant, with very large leaves. It occurs naturally from southern Mexico to southern Brazil. However, the bicolorous form, which tends to be purplish colored on the undersides of the leaves (M. magnifica Triana in horticulture), seems to be restricted to southern Mexico, Guatemala, Belize and Costa Rica.

Because of its large, colorful leaves, the M. calvescens was brought to Tahiti as an ornamental plant. The seeds were widely dispersed (presumably by birds) and the plant soon began prospering in its new home. Because it competes well with the native plants of Tahiti, M. calvescens has become an aggressive invader, displacing much of the native vegetation, and is now a serious threat to the native flora. This plant was later brought to Hawaii, where it has become one of the most serious threats to rainforests and agricultural areas, and in 1992 it was declared a noxious weed.

Populations of weeds are generally controlled by the application of herbicides, mechanical remova and/or biological control. The latter consists of utilizing insects and/or plant pathogens that feed on the weed, but do not feed on other plants, to reduce the population of the weed. Three factors make M. calvescens a perfect candidate for a biological control program:

1. M. calvescens is an exotic species in Hawaii. If an introduced species of organism becomes established in a new area, it is quite common that its population increases dramatically. In many cases this occurs because the organisms that normally keep the population in check, the “natural enemies”, were not introduced. In the absence of natrual enemies the population explodes. The underlying idea of biological control is to locate these natural enemies (or “biological control agents”) in the area where the weed is native, and introduce them into the area where the weed is causing problems.

2. Infestations in many parts of Hawaii are too widespread for effective chemical or mechanical control. These other control measures have to be frequently repeated and are therefore expensive, whereas a successful biological control program could potentially provide a permanent solution to the problem (natural enemies never totally eliminate the pest species, but often reduce its population to the point where it is no longer a pest). Chemical control of course entails potential environmental and public health risks, whereas successful biological control does not.

3. There are no native Melastomataceae in Hawaii, which means that potential control agents need only demonstrate specificity to the family level. It is quite likely that relatively few insects feed only M. calvescens, but there are undoubtedly many insects that feed only on M. calvescens and closely related melastomes.

The essential idea of this project – which is being carried out in collaboration between the University of Costa Rica, the University of Hawaii, and the U.S. Department of Agriculture – is to investigate the insects that feed on M. calvescens in Costa Rica, and to identify those species that show the greatest promise of being potential biological control agents.

But isn’t it risky introducing yet another exotic organism? This a common question and is an extremely important one. The short answer is that, if done correctly, no. If potential biological control agents are carefully screened, to ensure that they will not affect “non-target” organisms, the risks are minimal.

But can’t the introduced biological control agent “mutate” and begin feeding on other plants? Modern (scientific) biological control has been going on for over a hundred years and unpredicted host-switching has never occurred. Host-switching is nearly always predictable – one should never introduce species known or suspected of being generalists. Specialists on the other hand attack only a very restricted range of hosts. To ensure that only the latter are used, all potential biological control agents of Miconia calvescens will be carefully screened both in Costa Rica, and then again in Hawaii, under quarantine, before being released.

Biological control offers a potentially effective and environmentally sound solution to the Miconia calvescens problem in Hawaii and Tahiti. However, it should be clear from the above discussion that careful research is needed before carrying out such a program.

Authorship details
Hanson, P., 2003 (June 2). Introduction.
In Biological Control of Miconia calvescens. http://www.miconia.net