Biology Reference
In-Depth Information
in the death of a goat from suspected cardiac arrest 4 days after the last tissue treat-
ment, which seems to support evidence from Scandinavia that sheep avoid grazing
on PSW plants (DiTommaso et al 2005b; Haeggstrom 1990).
The New York State Forest Owner's Association and many foresters have
claimed that swallow-wort infestations in understories are also compromising
forest regeneration (Lawlor 2003). Horticultural nursery owners and Christmas tree
producers affected by swallow-wort infestations reported that due to lack of effec-
tive control methods and regeneration impacts, land abandonment was often the
only reasonable option. Indeed, several orchard owners east of Rochester, NY cited
PSW as their most problematic weed species (A. Fowler, personal communication;
Lawlor 2003).
The potential for both swallow-wort species to serve as fatal hosts for Monarch
butterflies (
Danaus plexippus
L.), a condition in which adults lay eggs on the plants
but the larvae do not survive, has been well reported (Casagrande and Dacey 2001;
DiTommaso and Losey 2003). Casagrande and Dacey (2007) found that in fields
with little or no common milkweed (
Asclepias syriaca
L. - the butterflies' normal
host species), the density of eggs found on BSW stems was five times greater than
that found in a more diverse old-field site with abundant common milkweed.
Although there have been studies that questioned whether swallow-worts play a sig-
nificant role as fatal hosts for Monarch butterflies (Mattila and Otis 2003), it is likely
that through the competitive displacement of common milkweed populations, the
two swallow-wort species could ultimately pose a serious threat to Monarch butter-
fly populations in infested areas (DiTommaso et al. 2005b; Tewksbury et al. 2002).
13.6 Management
13.6.1 ManualControl
Manual methods can often be effective at controlling established patches of peren-
nial weeds (Radosevich et al. 1997; Ross and Lembi 1999). However, both PSWs
and BSWs can rapidly regrow from buds on the root crown, rendering mowing,
tillage, clipping, and other frequently used control strategies less effective against
these perennials (Averill et al. 2008; Lawlor 2002; Lawlor and Raynal 2002;
Weston et al. 2005). Mowing can contain invasive populations of the swallow-worts
when timed to suppress seed production, but must be repeated for the duration of
the growing season as plants tend to regrow more rapidly than nonmowed plants
and produce seed at a more immature stage of growth than is typical (C.H.
Douglass, personal observations). Averill et al. (2008) found that clipping of PSW
stems once annually at the beginning of summer (June) led to a 44% reduction in
cover at an infested site in northern New York over a 2-year period. Because of their
tall, brittle stems, swallow-worts are also particularly sensitive to trampling, which
has resulted in a substantial reduction of PSWs in some localized fields (DiTommaso
et al. 2005b).
