Suhling et al. - 2000 - Effects of insecticide applications on macroinvert

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classified. The same was true <strong>of</strong> the corixid larvae.
Consequently sm<strong>al</strong>l odonate larvae and corixid larvae
were excluded from the an<strong>al</strong>ysis.
Results
<strong>Effects</strong> <strong>of</strong> sample date
A tot<strong>al</strong> <strong>of</strong> 84 species <strong>of</strong> macroinvertebrates was found
in the experiment<strong>al</strong> rice-fields. In May, 18 taxa were
found and the number increased to 40 in June and 56
in July, decreasing slightly in August to 53. In most <strong>of</strong>
the taxa, fewer than 30 individu<strong>al</strong>s were caught during
the sample periods. Twenty-three taxa were found in
abundance <strong>al</strong>lowing a statistic<strong>al</strong> comparison <strong>of</strong> the effects
<strong>of</strong> sample period and <strong>insecticide</strong> application on
densities. With the exception <strong>of</strong> the be<strong>et</strong>le Guignatus
pusillus (Fabr.) and the bugs Corixa punctata (Illiger)
and Sigara later<strong>al</strong>is (Leach) the densities <strong>of</strong> <strong>al</strong>l <strong>of</strong>
these 23 taxa varied significantly with the sample dates
(Table 1). Only the oligocha<strong>et</strong>es and the chironomids
reached their greatest abundance in May and June, respectively
(see Table 2). Gyraulis chinensis (Dunker),
Ba<strong>et</strong>is sp., Berosus ssp., Laccophilus sp. and the Hydrachnellae
were found in their highest densities in
July and <strong>al</strong>l the others occurred mainly in August,
particularly carnivorous species such as odonates and
some bugs (Notonecta, Naucoris).
Insecticide application and macroinvertebrate density
In nine <strong>of</strong> the 23 taxa an<strong>al</strong>ysed, no significant effects
<strong>of</strong> the treatment (<strong>insecticide</strong> application / non application)
were found (Table 1). These were Physella acuta
(Drap.), Oligocha<strong>et</strong>a, Hydrachnellae, Orth<strong>et</strong>rum <strong>al</strong>bistylum
(Sélys), Crocothemis erythraea (Brullé), C.
punctata, S. later<strong>al</strong>is, Chironomidae and G. pusillus.
In 14 taxa, two-way ANOVAs indicated significant effects
<strong>of</strong> the treatment and / or significant treatment ×
date interactions on density (see Table 2). We should
keep in mind that the first <strong>insecticide</strong> application was
carried out after the second sample period, so, differences
in density b<strong>et</strong>ween treatments not should
be expected before the sample period in July. Consequently
the macroinvertebrates may be divided into
four categories:
1. Taxa in which the densities were gener<strong>al</strong>ly lower
in the <strong>insecticide</strong> treatments than in the untreated
samples following the first <strong>insecticide</strong> application
in June, and significantly in at least 1 month.
This group includes three ephemeropterans (Ba<strong>et</strong>is
sp., Cloeon dipterum (L.), Caenis sp.), two
odonates (Orth<strong>et</strong>rum cancellatum (L.), Symp<strong>et</strong>rum
fonscolombii (Sélys)), the carnivorous H<strong>et</strong>eroptera
and three coleopterans (Berosus spp., H<strong>al</strong>iplinus
heydeni Wehnke, Noterus sp.) (see Table 2).
2. The Culicidae which occurred in significantly
lower density in the <strong>insecticide</strong> than in the non<strong>insecticide</strong>
treatment following the Icazon application
in June but in significantly higher density
following the Fastac application in July (Table 2).
3. Ischnura elegans (Vander L.) which, in contrast,
was found in significantly higher numbers in July
after the application <strong>of</strong> Icazon in the <strong>insecticide</strong>
treatment, but in significantly lower numbers in
August after the application <strong>of</strong> the pyr<strong>et</strong>hroid
(Table 2).
4. The fin<strong>al</strong> group includes Gyraulus chinensis
(Dunker), Erpobdella octoculata (L.) and Laccophilus
sp., in which the densities were higher in the
<strong>insecticide</strong> treatment than in the non-<strong>insecticide</strong>
treatment in July and August.
Results from odonate emergence versus sampling
A tot<strong>al</strong> <strong>of</strong> eleven species <strong>of</strong> odonates occurred in the
fields, and in 10 <strong>of</strong> these emergence was registered
(Table 3). Results from the emergence studies differed
in one aspect from those derived from sampling: in
I. elegans, there was no significant difference in the
number <strong>of</strong> emerged individu<strong>al</strong>s b<strong>et</strong>ween the two treatments
(Table 3). However, in the emergence <strong>of</strong> Anisoptera,
the results resemble those from the sampling:
exuviae <strong>of</strong> S. fonscolombii and O. cancellatum were
found in significantly higher numbers in fields without
<strong>insecticide</strong>s.
Macroinvertebrate community: biomass, density,
diversity
Tot<strong>al</strong> macroinvertebrate biomass and density increased
from May to July (Figure 1). In July both biomass
and tot<strong>al</strong> density were significantly higher in the
<strong>insecticide</strong>-treated fields than in the untreated fields.
In this month we found the maximum mean biomass
(dry weight) <strong>of</strong> about 3 g m −2 . In the other months
there were no significant differences. From July to August,
there was no increase in either param<strong>et</strong>er except
<strong>of</strong> the biomass in the untreated fields where it reached
its maximum mean v<strong>al</strong>ue in August.
To compare the community diversities b<strong>et</strong>ween the
sample dates and the <strong>insecticide</strong>-treated and untreated