Agriculture Reference
In-Depth Information
low-input than in the conventional treatments (Berkelmans
et
al
. 2003). Moreover, the struc-
ture index, a measure of the number of trophic layers and potential for regulation of oppor-
tunists, was generally higher in the organic and low-input than in the conventional systems
(Berkelmans
et
al
. 2003). Suppression of
Meloidogyne
javanica
in a bioassay was negatively cor-
related with the channel index, indicating that suppression was associated with a bacteria-
dominated food web as observed in the organic and low-input plots of the SAFS experiment
(Berkelmans
et
al
. 2003). A positive correlation between suppression of
M
.
javanica
and micro-
bial biomass had been described earlier (Jaffee
et
al
. 1998). At that time, no relationship was
found between
M
.
javanica
suppression and management system, nor with total number of
nematode trapping fungi, yet the diversity of nematode trapping fungi was greater in the
organic plots (Jaffee
et
al
. 1998).
Thus, the general tendency was that root infections by fungal pathogens and populations of
plant parasitic nematodes were lower in organic than in conventional soils, and this was asso-
ciated with either higher microbial diversity and activity, and/or a better soil structure, and/or
a more complex soil food web in the organic soils. These characteristics are typical for a healthy
soil that can resist disturbances by invading species (van Bruggen and Semenov 2000). Such a
healthy soil is attained by organic practices such as winter cover cropping, applications of
compost, and avoidance of synthetic pesticides and fertilisers.
Pest incidence, tomato injury and arthropod community structure
In the SAFS experiment, arthropod pests were monitored every two weeks by taking plant and
fruit samples. Pest populations f fluctuated significantly from year to year. Russet mites were
occasionally problematic, and were treated with sulfur in all fields in the first few years (Clark
et
al
. 1998). Potato aphids, armyworm and tomato fruit worms were severe enough to warrant
insecticide sprays in the conventional treatments. Insecticidal soap and Bt were occasionally
applied in the organic treatment. These were less effective than synthetic insecticides, so that
aphid and armyworm populations were sometimes higher in organic plots, but in general,
insect pests did not differ significantly among management treatments. The lack of significant
differences among treatments was attributed to the relatively small plot size (0.11 ha per plot),
necessitating an extensive on-farm field survey (Clark
et
al
. 1998).
In the field surveys, arthropods were vacuum extracted from tomato foliage. The major
tomato pests, such as thrips, aphids, tomato russet mite, f lea beetles, leaf-eating caterpillars,
leafminers, fruit-eating caterpillars and fruit-piercing insects were present in all fields. Average
damage levels accruing over the season were significantly correlated with the mean abundance
of the most common species of that pest group collected in vacuum samples. That is, western
f flower thrips
Frankliniella
occidentalis
abundance was directly correlated to percent tomato
leaf lets damaged by thrips; f lea beetle (
Epitrix
hirtipennis
) abundance was positively corre-
lated with percentage tomato leaf lets damaged by pit-feeders; and tomato fruitworm (
Helicov-
erpa
zea
) abundance and percentage of fruits with deep wounds typical of fruitworm damage
were also significantly correlated.
Damage from insect pests was variable among fields and among pest groups (e.g. leaf
grazers, foliage pit-feeders, fruit punctures), but the average levels of overall and specific types
of damage in organic and conventional fields were not significantly different (Drinkwater
et
al
. 1995, Letourneau and Goldstein 2001). The average abundance of phytophagous insects
was virtually the same on organic and conventional tomato at the time of crop harvest.
Although crop N levels were significantly lower in organic fields, neither pest levels nor damage
was explained by tissue N levels (Letourneau
et
al
. 1996).
However, community-level profiles (richness and abundance of herbivores and natural
enemies) in commercial tomato fields under organic and conventional management were
significantly different despite the wide range of specific farming practices and conditions
