**6. Conclusions**

country can be made for each insecticide. Since there is a log-to-log linear relationship be‐ tween concentration and time of exposure [234], the critical levels of residue and time of ex‐

The declining populations of predatory and parasitic arthropods after exposure to recom‐ mended applications of most systemic insecticides are worrying. In view of the above, it not so much the small concentrations they are exposed to but the time of exposure that makes the population decline progressively over weeks, months and even years of treatment, as described in this chapter. Lethal and sublethal effects on reproduction are equally implicat‐ ed. This is the reason why systemic insecticides should be evaluated very carefully before using them in IPM schemes. Obviously, recovery rates are essential for the populations af‐ fected to come back, and this usually occurs by recolonisation and immigration of individu‐ als from non-affected areas. For example, modelling based on recovery data after dimethoate application to wheat fields [277] demonstrates that a non-target organism that is reduced by only 20% but is unable to recover is likely to be far more at risk from exposure to a pesticide than an organism that is reduced 99% for a short period but has a higher recov‐

The above is also relevant to the impact of small residues of those systemic insecticides that have cumulative effects (e.g. neonicotinoids, fipronil and cartap) on aquatic ecosystems. Be‐ cause of the short life-cycle of many zooplankton species, the negative population parame‐ ters that result from sublethal and chronic effects on such organisms can lead their local populations to extinction [260]. Immediate reductions in populations and species may not always be apparent due to the small residue concentrations and the delayed effects they cause. For example, in recent surveys of pesticide residues in freshwaters of six metropolitan areas of USA, fipronil appears regularly in certain states [254]. Fipronil and its desulfinyl, sulfide, and sulfone degradates were detected at low levels (≤ 0.18–16 µg/L) in estuary wa‐ ters of Southern California [163], and make some 35% of the residues found in urban waters, with a median level of 0.2-0.44 µg/L, most frequently during the spring-summer season [99]. Imidacloprid was detected in 89% of water samples in agricultural areas of California, with 19% exceeding the US Environmental Protection Agency's chronic invertebrate Aquatic Life Benchmark of 1.05 µg/L [261]. In the Netherlands, imidacloprid appeared in measurable quantities in 30% of the 4,852 water samples collected between 1998 and 2007 [287]. These figures indicate there is already a widespread contamination of waterways and estuaries

The first consequence of such contamination is the progressive reduction, and possible elim‐ ination, of entire populations of aquatic arthropods from the affected areas. As time is a crit‐ ical variable in this type of assessment, it is envisaged that should this contamination continue at the current pace over the years to come the biodiversity and functionality of many aquatic ecosystems will be seriously compromised [191]. Secondly, as these organisms are a primary food source of a large number of vertebrates (e.g. fish, frogs and birds), the depletion of their main food resource will inevitably have indirect impacts on the animal populations that depend on them for their own survival. The case of the partridge in Eng‐ land is an example of how a combination of herbicides and insecticides can bring the demise

posure can be determined.

388 Insecticides - Development of Safer and More Effective Technologies

ery potential.

with persistent systemic insecticides.

This review has brought some light on the direct, sublethal and indirect effects that systemic insecticides have on species populations and ecosystems. Some long-term impacts have been known for some time (e.g. carbofuran, phorate), but it is the rapid increase in the usage of neonicotinoids and other systemic products that poses a new challenge to the ecological risk assessment of agrochemicals. Indeed, current risk protocols, based on acute, short-term toxic effects are inadequate to cope with the chronic exposure and cumulative, delayed im‐ pacts of the new compounds. Awareness of the increasing contamination of the environ‐ ment with active residues of these chemicals should help regulators and managers to implement new approaches for risk assessment of these substances.
