Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 377

1. The use of any one organism as a weed biocontrol agent should be integrated with
the use of other weed-suppressive herbivores , pathogens , competitors , and manage-
ment practices.
2. The possible impacts of introduced weed biocontrol agents on target and non-target
organisms should be rigorously evaluated and openly discussed before the agents are
released.
The first principle is directed at intensifying the impact of biocontrol agents
on target weed species, while the second is directed at using those agents pru-
dently.Throughout this chapter,our intent is to better understand the ecolog-
ical relationships underlying weed biocontrol, and to see where insect
herbivores and phytopathogens might provide farmers with practical benefits
in the near future.
Conservation of resident herbivores and pathogens
As noted in Chapters 2 and 5, weed density and growth can be
reduced by insects and microorganisms already present in agricultural habi-
tats and not intentionally introduced for weed suppression. In the absence of
other weed management tactics, populations of these organisms generally are
not capable of suppressing weeds to the point where crop yield loss is pre-
vented. They may contribute, however, to the regulation of weed populations
at several life stages. By better understanding the ecology of resident herbi-
vore and pathogen species that attack weeds, it may be possible to identify
management strategies that enhance their impact.
Weed seeds on the soil surface are vulnerable to predation by resident
insects, rodents, and other organisms. Carabid beetles are highly active on the
soil surface and may be particularly important as weed seed predators (Lund &
Turpin, 1977). Cardina et al . (1996) put 50 seeds of Abutilon theophrasti in soil-
filled petri dishes,placed them on the surface of Ohio corn fields,and observed
rates of seed removal of 1% to 57% per day.They calculated that at the average
removal rate of 11.2% per day, 80% of the seeds would be removed in four
weeks. Pitfall trapping, field experiments with protective cages made with
different mesh sizes, and laboratory feeding trials indicated that carabid
beetles, mice, and slugs were responsible for weed seed removal and destruc-
tion. In North Carolina soybean fields, Brust & House (1988) observed carabid
beetles, ants, crickets, and mice feeding on weed seeds. Over a five-week
period,seeds of four broadleaf annual weeds,placed on cards at densities of 25
to 50 seeds per card, were removed at rates of 2.2% to 4.2% per day. In Maine
potato and grain fields, carabid beetles were found to reduce or eliminate