Indirect interactions are almost inevitable in any multi-species community. Understanding the implications of such interactions is a challenging task, in light of the very large number of ways species can be tied together in complex food webs. One approach to this complexity is to focus on strong interactions among a relatively small number (e.g. 3-6) of species interacting in defined configurations: community modules. In recent years, the discipline of community ecology has developed a substantial body of theory focused on such modules. Modules often clearly describe the basic features of empirical systems, particularly in simplified anthropogenic landscapes, and also help to isolate and characterize key processes driving the dynamics of more complex communities. In this chapter, we draw out a number of insights from ecological studies of modules which we believe are relevant to biological control. We emphasize in particular the module of 'shared predation', where a natural enemy attacks two or more species of prey. Theoretical studies suggest a number of 'rules of thumb', including: (i) the greatest risk to non-targets may occur from control agents that are only moderately effective on the target; (ii) targets with a high reproductive capacity can indirectly endanger non-targets; (iii) there can be transient phases of extinction risk for non-targets during the establishment phase of control agents, particularly for species with high attack rates; (iv) at a landscape scale, mobile agents can endanger the fate of non-targets at sites other than the area of control; (v) using specialist natural enemies can pose risks to non-targets, if there are generalist resident predators/parasitoids which can exploit these introduced agents. The theoretical models help to highlight circumstances when these effects should be particularly strong.

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To estimate the potential growth and distribution of itchgrass (Rottboellia exaltata L. f.) in the United States, we grew the plant in 36 combinations of day and night temperatures in controlled-environment greenhouses. Day temperatures ranged from 17 to 32 C and night temperatures from 11 to 26 C. Total dry weights, leaf areas, heights, and numbers of tillers were determined at 3, 31, and 58 days after emergence. Mathematical growth analysis techniques were used to calculate net assimilation rates (NAR) and leaf area durations (LAD). Maximum dry matter production (the product of NAR and LAD) and maximum leaf area production occurred at 32/26 C. Dry matter production and leaf area production were greatly reduced by day temperatures below 29 C and night temperatures below 23 C. The retardation of dry matter production by low night temperature was due mainly to reductions in LAD rather than to reductions in NAR. Low day temperatures reduced dry matter production through combined effects on LAD and NAR. At the warmer day temperatures, tiller production increased with decreasing night temperature. Flowering occurred at all day temperatures and at all night temperatures of 14 C or greater. Based on its growth responses to temperature, itchgrass would reach 75 to 100% of its maximum potential growth in the Gulf Coast states, the lower Midwest, the South Atlantic states, and the Southwest. Therefore, it represents a serious potential weed problem in these regions.

In greenhouse experiments, conidia ofColletotrichum truncatumapplied in an invert emulsion formulation controlled hemp sesbania 100% in the absence of a dew treatment. In field experiments, hemp sesbania control averaged 95 and 97% in 1989 and 1990, respectively, when this formulation was applied to hemp sesbania seedlings using tractor-mounted, air-assist nozzles. This level of weed control was comparable to that achieved from the herbicide acifluorfen. These results indicate thatC. truncatumhas excellent potential as a mycoherbicide for controlling hemp sesbania and that this potential can be augmented by formulating the pathogen as an invert emulsion.

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The spread of integrated pest management (IPM) practices and concern for the environmental impact of alien invasive species will increase demand for biological control in the next century. Meeting the demands of a broader stakeholder community will require further development and harmonization of international protocols, with greater attention to non-target, ecological effects. Ecological theory and research has a key role to play in delivering tools to evaluate these effects, thereby improving efficacy and safety of biological control.

Applications of the fungus, Alternaria cassiae Jurair and Khan with and without an overspray of a new experimental invert (water-in-oil) emulsion were evaluated for control of sicklepod (Cassia obtusifolia L.) weed in soybeans ‘Forrest’ with and without a post-treatment dew period under controlled environmental conditions. The unique experimental invert emulsion was developed and applied with specialized air-assist atomizing nozzles as an overspray to provide moisture and to retard evaporation of the water applied so that the fungal spores could germinate and infect the target weeds under relatively dry conditions. Spores applied in an aqueous carrier alone or in an aqueous carrier followed by an overspray of the invert emulsion caused 100 percent mortality by 16 days after treatment if the sicklepod seedlings were exposed after treatment to 18 h of dew. Spores applied in the aqueous carrier alone and in the aqueous carrier followed by an overspray of the invert emulsion caused 0 and 88 percent mortality, respectively, without the dew period. This is the first report known to us of a technique to obtain infection and weed control with a foliar-applied weed pathogen without dew or mist.

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