Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 482

Moreover, reducing dispersal between fields restricts the entry of new
genetic variability into weed populations. The utility of not allowing immi-
gration of herbicide-resistant biotypes into populations is well understood
(Darmency, 1994; Morris, Kareiva & Raymer, 1994; Jasieniuk, Brûlé-Babel &
Morrison,1996),but entry of other sorts of genetic variability is often undesir-
able as well since it may provide the raw material for future adaptation of the
population. Many self-compatible weed species have low within-population
isozyme polymorphism but substantial variation among populations
(Barrett, 1988; Wang, Wendel & Dekker, 1995 a , 1995 b ). As discussed previ-
ously, this probably reflects past genetic bottlenecks during colonization.
Assuming the bottlenecks have had similar effects on adaptive variation,
further adaptation at the local level may depend on arrival of genetic material
from outside the population.
Methods for preventing the spread of weeds in manure, on machinery, in
crop seed,etc.were detailed in Chapter 2.In addition,since a large proportion
of the propagules dispersing between fields in a district in any given year
probably originate from a relatively few heavily infested fields, control of
weeds at those locations will reduce spread. Finally, if the newly arrived
species or problem biotype can be recognized soon after arrival, it can be erad-
icated while the population is small and localized.
The problem is to implement all these methods through education and
institutional response. This may be accomplished through a three-pronged
approach using the participatory learning-for-action model discussed in
Chapter 3. In some respects the proposed approach resembles quarantine and
eradication programs used against particularly noxious weeds (e.g., Striga
asiatica in the USA - Sand, 1987). However, for most weeds such extreme
measures are not cost-effective. Rather, mechanisms are needed for co-
ordinating the collective good sense of the agricultural community for
regional management of emerging weed problems.
First, the epidemiology of major weeds and potentially serious new weeds
and weed races in a region needs to be elucidated. In particular, how is each
taxon spreading among farms? Are additional mechanisms promoting spread
among fields within farms? What is the relation between the number of prop-
agules in an inoculum and the probability of establishing a population? What
is the distribution of infested fields in the region? Where are the fields with
extreme infestations located? These questions need to be answered first for
those species and biotypes that are most potentially damaging to the agricul-
ture of the region. With time, databases for additional species can be added.
Understanding the mode of spread and effective inoculum size will generally
require a focused, but not necessarily extensive, research effort. Information
on distribution of problem weeds could be collected by extension agencies