Pesticide Residues in Soil Invertebrates - Pesticides: Evaluation of Environmental Pollution - page 350

Reductions in populations of up to 90% were observed in some trials. In some cases, this
decrease in predatory beetles was associated with the increased incidence of aphids, one
of their major food sources (Al-Haifi et al. 2006).
The significance of predators, such as spiders, in controlling pests in agricultural eco-
systems (Marc et al. 1999) and as an important food source for farmland birds has been
recognized (Wilson et al. 1999). In addition, the importance of Collembola for increas-
ing fungal decomposition (Cragg and Bardgett 2001) and maintaining spider numbers
when pest populations are low (Marcussen et al. 1999; Bilde et al. 2000; Harwood et al.
2001, 2003, 2004; Agusti et al. 2003) has highlighted the need for more studies investi-
gating mechanisms for soil invertebrate population regulation. Therefore, the effects
of the insecticide, chlorpyrifos, on spider and Collembola communities were studied.
Initially, twelve species of Collembola were identified from the insecticide-treated
and control plots. Species diversity, richness, and evenness were all reduced in the
chlorpyrifos-treated plots, although the total number of Collembola species increased
tenfold despite the abundance of some spider species being reduced. The dominant
collembolan in the insecticide-treated plots was Ceratophysella denticulata , accounting
for over 95% of the population. Forty-three species of spider were identified. There
were a reduced number of spiders in insecticide-treated plots due mainly to a lower
number of the linyphiid Tiso vagans . Immature Collembola and spiders were either
absent or in low numbers on the insecticide plots, respectively. Juveniles are often
more sensitive to xenobiotics than adults (Fountain and Hopkin 2001), and it is possible
that adults are better at detecting and avoiding insecticides (Fabian and Petersen 1994).
It emphasizes the importance of understanding the effects of soil management prac-
tices on soil biodiversity, which is under increasing pressure from land. Chlorpyrifos
is toxic to spiders (Pekár 2002) and also reduces other beneficial predators such as
Staphylinidae (Wang et al. 2001) and Carabidae (Curtis and Horne 1995).
12.3.6 Effect of Pesticides on Beneficial Soil Microorganisms
One spoonful of healthy soil has millions of tiny organisms including fungi, bacteria,
and a host of others. These microorganisms play a key role in helping plants utilize the
soil nutrients needed to grow and thrive. Microorganisms also help the soil store water
and nutrients, regulate water flow, and filter pollutants (Pell et al. 1998). The heavy
treatment of soil with pesticides can cause populations of beneficial soil microorgan-
isms to decline. According to soil scientist Dr. Elaine Ingham, “If we lose both bacte-
ria and fungi, then the soil degrades. Overuse of chemical fertilizers and pesticides
have effects on the soil organisms that are similar to human overuse of antibiotics.
Indiscriminate use of chemicals might work for a few years, but after awhile, there
aren't enough beneficial soil organisms to hold onto the nutrients” (Santos and Flores
1995). For example, plants depend on a variety of soil microorganisms to transform
atmospheric nitrogen into nitrates that plants can use. Common landscape herbicides
disrupt this process: triclopyr inhibits soil bacteria that transform ammonia into nitrite
(Moorman et al. 1992); glyphosate reduces the growth and activity of both free-living
nitrogen-fixing bacteria in soil (Fabra et al. 1997) and those that live in nodules on plant
roots (Arias et al. 1993); and 2,4-D reduces nitrogen fixation by the bacteria that live
on the roots of bean plants (Singh and Singh 1989; Tözüm-Çalgan and Sivaci-Güner
1993), reduces the growth and activity of nitrogen-fixing blue-green algae (Martens and
Bremner 1993; Frankenberger and Tabatabai 1991), and inhibits the transformation by
soil bacteria of ammonia into nitrates. Mycorrhizal fungi grow with the roots of many