Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 68

Table 2.8. Survival of seedlings emerging in maize in July from establishment to
maturity (or for some A.theophrasti, death by hard frost)

Species Year Till No-till Rye Till Rye No-till

Amaranthus retroflexus 1986 0.27 0.44 0.25 0.46

1987 0.33 0.55 0.20 0.55

1991 0.78 0.82

1992 0.58 0.49

Chenopodium album 1986 0 0 0.15 0

1987 0.61 0.41 0.43 0.33

Portulaca oleracea 1986 0 0.04 0 0

1987 — 0.31 — —

Digitaria sanguinalis 1986 — 0.81 — 0.58

1987 — 0.84 — 0.79

1991 0.78 0.70

1992 0.60 0.16

Abutilon theophrasti 1991 0.94 0.96

1992 0.63 0.48

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Mohler (unpublished).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 matura-
tion. 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 of weed control practices create large
fluctuations in the density of particular weed species.Moreover,weed popula-
tions 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