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Sampling bias in ecotoxicological field research
In most field trials it is not the abundance but the activity abundance AA (= 'Aktivitätsdichte' sensu Heydemann, 1953; = 'availability' sensu Southwood 1978) of organisms which is recorded. Changes in AA are considered pesticide-impacted if the ratio to the control-AA before a treatment differs significantly from that after (cf., Everts 1990, Stewart-Oaten et al. 1986, Weyrich 1993). One of the main difficulties is to distinguish between random and non-random variations in AA. In the field experiments in Niger (Peveling and Weyrich 1992a) it was noticed that the AA of larger tenebrionids in the field varied in the same synchronous cycles as the relative activity RA of caged Pimelia angulata tschadensis. This cyclical, non-random but also non-insecticide-induced activity pattern was also observed in P. raffrayi raffrayi in Sudan (Peveling and Weyrich 1992b) and in P. subquadrata chudeaui (Fig. 7) in
Mauritania (Peveling 1993b), giving rise to the question of which factor causes the synchronisation of activity and what the methodological consequences might be.
Figure 7. Relative activity RA in field-caged Pimelia subquadrata chudeaui over a 17 days observation period. The RA on day i is the percentage of surviving beetles on day i caught in a set of pitfall traps opened daily for one hour (19.00-20.00 h). Figures are the mean incl. SD of three equally wind-exposed replicates (100 beetles in each replicate)
Climatological measurements during pitfall trap sampling revealed that wind velocity V was the prime factor controlling the RA of P. subquadrata chudeaui (Fig. 8). The RA at V > 1 m/s (max. = 4 m/s) was, on average, 27% higher than the RA at V < 1 m/s (t-Test, p < 0.01). Temperature T and relative humidity RH had no significant influence, despite their great range (T: 20.0-28.5° C; RH: 15-46%). Thus between-day activity variations reflected for the most part cyclical changes in wind velocity. This „wind control" of locomotory activity can be readily explained through stimulation of sensory hairs.
Figure 8. Relative activity RA in relation to wind velocity V in a single group of field-caged Pimelia subquadrata chudeaui over a 17 days observation period. The broken line denotes the group median M. Note that on all but four days signs for V and RA are tied (bold print). + indicates V = 1 m/s and RA > M; - indicates V < 1 m/s and RA < M
It is evident that changes in locomotory activity inherently alter the probability of being trapped. Therefore, when sampling ground-dwelling arthropods subject to „wind control", total pitfall trap catches in different locations may simply differ because of the varying wind regime rather than pesticide impact. In practice, several basic conclusions can be drawn. If capture intervals in treated and untreated plots are not equal in length and synchronous, or when the activity cycles of certain taxa are non-synchronous due to different microclimatic conditions, i.e. phase shifted or running with different periods, biased or even incorrect results may be achieved. Thus, pesticide effects can be erroneously found where none exist or existing effects can be masked by „interference". This also applies to data analysis based on the principle of „pseudo-replicates in time" (Stewart-Oaten et al. 1986). As a consequence, processing data as if samples had been collected synchronous when in fact they had not (i.e. traps emptied on different days), a common practice for technical reasons (Keith 1992, van der Valk 1990), has to be rejected.
Discussion and prospects
The present investigations did not reveal severe side-effects of alternative control agents on epigeal non-target arthropods.
This was especially true for botanicals. Neither the repellency of neem to tenebrionids nor the retardation of reduviid larval development by Melia volkensii can be regarded as critical when compared to toxic effects of conventional insecticides. In general, side-effects of neem on beneficials in tropical agro-ecosystems have already been investigated extensively (Schmutterer 1990). Circumstantial evidence shows that predatory mites and parasitoids, e.g. trichogrammatids, can be adversely affected by neem. This is, however, more likely to be due to physical effects of the oil than to the toxicity of the active ingredients (Schmutterer 1993). Selecting for neem oils with a high natural azadirachtin content or using enriched oils will help to reduce the spray volume and thus minimise these physical effects. In contrast with neem, there is no information on side-effects of M. volkensii, and more extensive research is needed before it finds its place in locust and grasshopper control.
Though IGRs act primarily on phytophagous insects, beneficials, e.g. coccinelids, chrysopids, syrphids and aphids, are also affected (Boness 1983, Darwish and Farghal 1990, EI-Din et al. 1990). As for adverse effects on parasitoid hymenopterans like braconids and trichogrammatids results are divergent (e.g. Broadbent and Pree 1984, Hagley and Laing 1989, Narayana and Babu 1992, Retnakaran et al. 1985). Likewise, our own experiments gave an inconsistent picture: triflumuron for instance was clearly toxic to Coranus arenaceus, while in spiders some species were sensitive and others were not. Thus follow-up bioassays are necessary, and long-term effects of locust control on non-target populations and communities in terrestrial ecosystems should be monitored in more detail. Emphasis should be placed on aspects such as IGR-interference with fecundity (cf., Banks et al. 1988, Hammann and Sirrenberg 1980, Hejazi and Granett 1986, Reimer et al. 1991, Weaver and Begley 1982), alterations of trophic structure, and the fate and behaviour of IGRs in the environment.
The host range of entomopathogenic fungi is generally quite wide (Ferron 1985, Krieg and Franz 1989). Even so the search goes on for pathotypes which are more or less specific to acridids (Prior and Greathead 1989). Experience on side-effects has been for the most part gained in temperate environments (Baltensweiler and Cerutti 1986), e.g. during fungal control of the European cockchafer (M. melolontha) in Switzerland (Keller 1992). For tropical or sub-tropical, and certainly for arid environments, comparable experience is lacking. In Niger, in an extremely arid environment (Tmax > 40° C; RH < 20%), the pathogenicity of B. bassiana to tenebrionids was small. In contrast, under more favourable climatic conditions in Mauritania (Tmax < 35° C; RH > 35%), B. bassiana was highly virulent and infected individuals of all test species. Hence, target-specificity is out of the question here, and with a prolonged persistence - as is being sought through an improved conidia-formulation - greater side-effects can be expected. The high sensitivity of Peucetia sp. is notable. While spiders of temperate regions, thanks to fungicidal secretions (Holl pers. comm.), are not susceptible to fungal diseases, species adapted to arid conditions, where fungi obviously do not represent a selective factor, presumably do not possess such a defense mechanism. It is worthwhile investigating, whether arthropods from arid and semi-arid environments have in general developed less resistance against entomopathogenic fungi than their counterparts in temperate zones.
Despite the open questions remaining, there are at present no reservations against introducing alternative agents into integrated grasshopper and locust control. They should now be tested on a larger scale as substitutes for conventional insecticides in order to gain more field experience on both effects and side-effects. With respect to conventional control operations, in general more emphasis should be laid on the planning and implementation of environmental monitoring programmes.
As well as evaluating the ecotoxicological risks of alternative control agents to non-target arthropods, the present study gave an account of possible sampling bias in ecotoxicological field research. It was shown that non-synchronous sampling intervals can cause systematic errors in data processing. Even in a fully standardised experimental design there are many factors besides insecticide impact which might evoke different AA-responses in different plots, e.g. microclimatic variations in time and space, different trophic situations etc. As adverse effects on ecosystems may manifest themselves as disturbances either of structure (species composition, biomass, density etc.) or of function (mineralisation, biodegradation, natural mortality), the basic question arises whether the latter is a more reliable indicator of pesticide effect than the former. Concentrating on function, an approach recently pursued by Niassy et al. (1993), appears to be especially justified in integrated locust control, since structural effects are less obvious - and usually delayed - than in conventional locust control.
We thank the research institutions and corporations mentioned above for providing the agents tested. We are grateful to Dr. F. Saba, Bayer Corp., for performing residue analyses for treatment controls, Prof. A. Holl, Giessen, for critical discussion on the ecology of spiders, Mr M. Lillig, Saarbrücken, for the determination of Tenebrionidae, and Dr. A. Hänggi, Basel, for the preliminary determination of Mauritanian spiders. We express our special thanks to all colleagues who supported our practical work in Africa, especially Mr J. Haag (Niger), Mr G. Hanrieder (Sudan), Mr H. Abbas (Sudan), Mr J.M. Magzoub (Sudan), Mr J.-U. Heckel (Cape Verde), Mr V. Leffler (Mauritania) and Mr A.S. Seme (Mauritania). We also thank Ms F. Casci, London, for reviewing the manuscript.
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