11.2.6. Insects as biological control agents for weeds
Weeds are simply plants that are growing where they are not wanted. Some weed species are of little economic or ecological consequence, whereas the presence of others results in significant losses to agriculture or causes detrimental effects in natural ecosystems. Most plants are weeds only in areas outside their native distribution, where suitable climatic and edaphic conditions, usually in the absence of natural enemies, favor their growth and survival. Sometimes exotic plants that have become weeds can be controlled by introducing host-specific phytophagous insects from the area of origin of the weed. This is called classical biological control of weeds and it is analogous to the classical biological control of insect pests (as explained in detail in section 16.5). Another form of biological control, called augmentation (section 16.5), involves increasing the natural level of insect enemies of a weed and thus requires mass rearing of insects for inundative release. This method of controlling weeds is unlikely to be cost-effective for most insect—plant systems. The tissue damage caused by introduced or augmented insect enemies of weeds may limit or reduce vegetative growth (as shown for the weed discussed in Box 11.3), prevent or reduce reproduction, or make the weed less competitive than other plants in the environment.
A classical biological control program involves a sequence of steps that include biological as well as sociopolitical considerations. Each program is initiated with a review of available data (including taxonomic and distributional information) on the weed, its plant relatives, and any known natural enemies. This forms the basis for assessment of the nuisance status of the target weed and a strategy for collecting, rearing, and testing the utility of potential insect enemies. Regulatory authorities must then approve the proposal to attempt control of the weed. Next, foreign exploration and local surveys must determine the potential control agents attacking the weed both in its native and introduced ranges. The weed’s ecology, especially in relation to its natural enemies, must be studied in its native range. The host-specificity of potential control agents must be tested, either inside or outside the country of introduction and, in the former case, always in quarantine. The results of these tests will determine whether the regulatory authorities approve the importation of the agents for subsequent release or only for further testing, or refuse approval. After importation, there is a period of rearing in quarantine to eliminate any imported diseases or parasitoids, prior to mass rearing in preparation for field release. Release is dependent on the quarantine procedures being approved by the regulatory authorities. After release, the establishment, spread, and effect of the insects on the weed must be monitored. If weed control is attained at the initial release site(s), the spread of the insects is assisted by manual distribution to other sites.
There have been some outstandingly successful cases of deliberately introduced insects controlling invasive weeds. The control of the water weed salvinia by a Cyrtobagous weevil (as outlined in Box 11.3), and of prickly pear cacti, Opuntia species, by the larvae of the Cactoblastis moth are just two examples. On the whole, however, the chances of successful biological control of weeds by released phytophagous organisms are not high (Fig. 11.6) and vary in different circumstances, often unpredictably. Furthermore, biological control systems that are highly successful and appropriate for weed control in one geographical region may be potentially disastrous in another region. For example, in Australia, which has no native cacti, Cactoblastis was used safely and effectively to almost completely destroy vast infestations of Opuntia cactus. However, this moth also was introduced into the West Indies and from there spread to Cuba and Florida, where it has increased the likelihood of extinction of native cactus species, and it now threatens North America’s (and Mexico’s) unique cacti-dominated ecosystems.
In general, perennial weeds of uncultivated areas are well suited to classical biological control, as long-lived plants, which are predictable resources, are generally associated with host-specific insect enemies. Cultivation, however, can disrupt these insect populations. In contrast, augmentation of insect enemies of a weed may be best suited to annual weeds of cultivated land, where mass-reared insects could be released to control the plant early in its growing season. Sometimes it is claimed that highly variable, genetically outcrossed weeds are hard to control and that insects “newly associated” (in an evolutionary sense) with a weed have greater control potential because of their infliction of greater damage. However, the number of studies for which control assessment is possible is limited and the reasons for variation or failure in control of weeds are diverse. Currently, prediction of the success or failure of control in terms of weed or phytophage ecology and/or behavior is unsatisfactory. The interactions of plants, insects, and environmental factors are complicated and likely to be case-specific.
In addition to the uncertainty of success of classical biological control programs, the control of certain weeds can cause potential conflicts of interest. Sometimes not everyone may consider the target a weed. For example, in Australia, the introduced Echium plantagineum (Boraginaceae) is called “Paterson’s curse” by those who consider it an agricultural weed and “Salvation Jane” by some pastoralists and beekeepers who regard it as a source of fodder for livestock and nectar for bees. A second type of conflict may arise if the natural phytophages of the weed are oligophagous rather than monophagous, and thus may feed on a few species other than the target weed. In this case, the introduction of insects that are not strictly host-specific may pose a risk for beneficial and/or native plants in the proposed area of introduction of the control agent(s). For example, some of the insects that can be or have been introduced into Australia as control agents for E. plantagineum also feed on other boraginaceous plants. The risks of damage to such non-target species must be assessed carefully prior to releasing foreign insects for the biological control of a weed. Some introduced phytophagous insects may become pests in their new habitat.
The data include 72 weed species that have agents introduced and established long enough to permit control assessment. (After Sheppard 1992; based on data from Julien 1992)