FEATURE ARTICLE

Where Does Biological Control Fit in Row-Crop Weed Management?

Biological control of weeds has been successfully used in perennial cropping systems such as rangeland [i.e. St. Johnswort (Hypericum perforatum) control using the beetle, Chrysolina quadrigemina] and orchards [Phytophthora palmivora (Devine) for control of strangler vine (Morrenia odorata) in citrus] or in small grain crops such as rice [Colletrotrichum gloeosporioides F. sp. aeschynomene (Collego) for control of northern joint vetch (Aeschynomene virginica)] but there has been limited success in corn and soybean systems.

Does biological control of weeds have a place in these row-crop systems? Will these cropping systems continue to rely predominately on chemical herbicides for weed control? Will biocontrol always be marginal, not having a noticeable impact on weed management in large acreage crops in the Midwest? How will the shift to broad spectrum postemergence herbicides (i.e. glyphosate) and herbicide resistant corn and soybeans effect the introduction of biological control systems in these crops? These questions are difficult to answer and cloud the future of corn and soybean weed management using biocontrol agents.

Why has the adoption in the Midwest of biological control systems for row crops been so limited? I feel that funding and weed scientist knowledge have had impacts on adoption of biological control. Research funding for biocontrol has generally been limited, especially compared to industry funding available to evaluate herbicides. Many applied weed scientists-those developing weed management recommendations-do not have adequate training in ecology, entomology, and plant pathology. While there is an active group of weed scientists working on biocontrol, their impact on current soybean and corn weed management systems has been limited. This is reflected in Weed Science Society of America (WSSA) publications, where there have been few recent articles on biological control of weeds compared to articles on weed control using herbicides. In the past two years in Weed Technology, the applied journal of the WSSA, there have been 123 articles dealing with herbicides and only 8 biocontrol articles.

I feel that adoption of biocontrol in row crop systems has also been limited by some of the characteristics of biocontrol agents. Classical biological control agents such as herbaceous insects or plant pathogens are only effective against a single species or at best a few closely related species and there has been research on biocontrol agents for single species of common corn and soybean weeds. But the weed complex found in row crops such as corn and soybeans consists of many annual weeds and if a single species is controlled other species will replace it. We may be able to increase the spectrum of weeds by combining biocontrol agents or tank mixing biocontrol agents with herbicides. But much more research will be required. Biocontrol agents may have an important fit in controlling weed biotypes with resistance herbicides such as those inhibiting acetolactate synthesis (ALS). As Roundup-Ready soybeans and corn are more widely used, biocontrol agents might become important for managing difficult to control weeds, such as nightshades (Solanum spp.) and perennial weeds such as Canada thistle (Cirsium arvense), in those systems.

Production systems with allelopathic crops or rotational cover crops may be a way to manage a range of weed species using a biological control approach. Many plants release chemicals which suppress a wide spectrum of weeds. There has been limited research on allelopathic row crops and no attempts to release corn or soybean cultivars with high levels of allelopathy. Even if allelopathic row crops are developed, allelochemicals will not control all weed species. For example, in my research, allelopathic cereal rye systems do not control seedling dandelion (Taraxacum officinale) and seedling perennial sowthistle (Sonchus arvensis). Allelochemicals are also unlikely to control perennial weeds. Another hurdle is that allelopathic crops must control weeds through the critical weed-free period which can be as much as 12 weeks and can not reduce yields or delay maturity of the desirable crop. Allelopathic cropping systems are probably best considered a replacement for preemergent herbicides. These systems may require postemergent herbicide applications to provide season-long weed control.

I also feel that the greater risk and potentially greater cost in using biocontrol agents compared to herbicides has limited their adoption for managing weeds in corn and soybeans. Environmental conditions will have a greater impact on biocontrol agents than they will on herbicide activity. Because we are dealing with annual cropping systems, biological control agents must be reintroduced each year. Biocontrol agents are especially sensitive to environmental conditions during establishment. It might be possible to develop formulations of biocontrol agents which reduce this problem. Will biocontrol agents be able to be used over wide areas or will they be site sensitive? How will the cost of weed management systems using biocontrol compare to systems relying solely on herbicides? I feel that cost might become an issue if a biocontrol system must be developed to control a wide spectrum of weeds.

The slower rate and the lower level of weed control with some biocontrol agents may also be a limitation. Will the time required for a biocontrol agent to build up to the levels necessary to suppress weeds be acceptable in row crops? Would combining bio-control agents with herbicides or using multiple biocontrol agents overcome this problem? Farmers generally aim to maximize weed control and thresholds are not widely used. Many common herbicides provide greater than 95% control of most annual weeds in corn and soybeans. Are these levels of weed control necessary? Will farmers accept lower levels of control and will they be willing to adopt thresholds? Can biocontrol agents provide levels of weed management similar to commonly used herbicides?

If inundative approaches to biological control are widely adopted in corn or soybeans who will profit? Commercialization is important for widespread use and adoption of biocontrol agents, especially those which must be reestablished each year. Only certain types of biocontrol agents, i.e. pathogens and nematodes, are likely to be successfully commercialized. Biotechnology may increase the range of biocontrol agents commercialized. But is a herbicide whose active ingredient is modified from a chemical identified as a fungal toxin still "biological control"? Will commercialization of the biocontrol agents be done by same companies producing herbicides? Can "small" companies compete? Will the return on a biocontrol product be greater than the cost for research, development, and production? How we answer these questions will determine the future of biocontrol of weeds.

- John Masiunas, University of Illinois


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