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No-till

Notes:

No comparisons were significant except for till vs. no-till for A.retroflexus in 1986 and 1987, C. album in 1987,and D.sanguinalis in 1992.

Source: Data for 1986 and 1987 from Mohler & Callaway (1992); data for 1991 and 1992 from C. L. Mohler (unpublished).

No-till

Notes:

No comparisons were significant except for till vs. no-till for A.retroflexus in 1986 and 1987, C. album in 1987,and D.sanguinalis in 1992.

Source: Data for 1986 and 1987 from Mohler & Callaway (1992); data for 1991 and 1992 from C. L. Mohler (unpublished).

Drought occasionally causes substantial mortality during the growth phase in some weed populations (Blackman & Templeman, 1938), but based on the several studies cited above, it does not appear to be a major limiting factor for most annual weeds. Usually drought will have greatest effect on population size during establishment rather than during growth and maturation. Once the weed has a well-established root system, a drought that kills many weeds is likely to severely damage the crop as well.

For introduced weed species, the low mortality rates may be partially due to escape from host-specific natural enemies. However, even in their native range most annual agricultural weeds probably escape serious attack because they represent unpredictable and ephemeral resources (Feeny, 1976). Crop rotation and year-to-year variation in the success ofweed control practices create large fluctuations in the density of particular weed species. Moreover, weed populations tend to be patchy (e.g., Wilson & Brain, 1991; Cardina, Sparrow & McCoy, 1996), and because they are usually mixed in with a larger population of a more dominant plant species, namely the crop, they are probably hard for host-specific herbivores to locate (Root, 1973). In an exceptional case, monarch butterfly larvae defoliated 31% to 78% of young Asclepias syriaca in soybean in Minnesota (Yenish et al., 1997). However, defoliation by monarch larvae was generally much less in maize due to lower weed seedling density, and in wheat because of a denser canopy during egg-laying by the adults. Thus, temporal variability, patchiness, and interference by the crop with search behavior of host-specific herbivores mitigate against effective control of weeds by these agents in annual crops. These obstacles could be overcome by mass application of host-specific herbivores. Mass application of herbivores for weed control has not been tried on a field scale, though Kremer & Spencer (1989a, 1989b) studied a seed-feeding scentless plant bug on Abutilón theophrasti for that purpose.

Many annual weed species, including Amaranthus retroflexus and Chenopodium album, are highly palatable to humans, indicating that they have poor physical and chemical defenses against generalist herbivores and pathogens (Feeny, 1977). However, management practices, especially tillage, tend to reduce populations of generalist enemies like mollusks (Hunter, 1967). Moreover, the growth rate of annual weeds is so high that once they are beyond the seedling stage they often increase in biomass faster than the herbivores can feed. Thus, once they have established, annual weeds usually escape control by generalist natural enemies as well as host-specific ones.

The generally small response of survival of annual weeds to variation in tillage and mulch (Table 2.8) indicates that manipulation of naturally occurring populations of herbivores and pathogens probably has limited potential for post-emergence weed management in annual cropping systems. This contrasts with the substantial management potential inherent in naturally occurring seed predators (see Chapter 8). Nevertheless, naturally occurring herbivores and pathogens may provide significant weed control in some systems, and these cases need to be identified, and programs developed that exploit this potential. For most annual weeds, the most effective approach to use of natural enemies for post-emergence control will usually be inundative release ofpathogens (Chapter 8).

Survival rates of perennial weeds during the period from seedling to first reproduction are typically much lower than for annuals. For example, Mortimer (1976) established four perennials in small plots in grassland (type unspecified) that was (i) turned with a spade, (ii) killed with herbicides and the dead surface vegetation removed, or (iii) clipped to 7.5 cm and then left undisturbed. Proportion of plants surviving over an eight-month period ranged from 0.01 to 0.18 (Figure 2.4). Poa annua and Plantago lanceolata survival was several-fold lower in the undisturbed grassland than in the plots with inverted soil. In nearly every case, exclusion of invertebrates with insecticides and molluskicides increased survival, although many of the differences were too small to be significant individually (Figure 2.4). Other studies have found

Figure2.4 Survival of four perennial species from seedling to adult during an eight-month period (September to May) in three grassland disturbance conditions in northern Wales, UK. (After Mortimer, 1976.)

similarly low or even lower survival rates for perennial weeds in perennial vegetation (Forcella & Wood, 1986; McEvoy et al., 1993; Qi, Upadhyaya & Turkington, 1996). In contrast with annual weeds, perennial weeds in perennial vegetation like pastures and hay meadows remain in the same location for an extended period. This makes them easier for herbivores to find and allows the build-up of enemies to effective levels. Even more importantly, the dense, diverse crop provides shelter for the herbivores that attack young weeds, and the shady, moist conditions within the sward facilitate attack by pathogens. In addition, the weed suffers competitive pressure from the crop, and in pastures suffers from grazing and trampling by livestock. These factors can be manipulated to decrease weed survival in perennial systems (Chapters 8 and 9).

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