Interesting question...Here is an excellent review on the issue, abstract is enclosed...

Effects of Pesticides on the Growth and Reproduction of Earthworm: A Review

 Abstract: :Scientific literature addressing the influence of pesticides on the growth and reproduction of earthworm is reviewed. Earthworms are considered as important bioindicators of chemical toxicity in the soil ecosystem. Studies on this aspect are important because earthworms are the common prey of many terrestrial vertebrate species such as birds and small mammals, and thus they play a key role in the biomagnification process of several soil pollutants. Majority of the studies have used mortality as an endpoint rather than subtler endpoints such as reproductive output. It is now emphasized that, whereas higher concentrations of a pollutant can easily be assessed with the acute (mortality) test, contaminated soils with lower (sublethal) pollutant concentrations require more sensitive test methods such as reproduction test in their risk assessment. source ;Applied and Environmental Soil Science

Volume 2010, Article ID 678360, 9 pages doi:10.1155/2010/678360

I think that the diffusion of chemicals into the soil by infiltration or by leaching, affect the fauna of the soil especially in the tropical environment or the intensity of the rains accelerates the diffusion of the pesticides.

Negative impact of pesticides on earthworm growth and development has been reported under tropical condition. The population of earthworm increase with increase in soil organic matter level and decrease with nitrates and pesticides with groundwater. For more details please follow the link below:

1. https://research.VU.nl/files/2538004/9090.pdf by P De Silva (2009)

2. https://www.hindawi.com/journals/aess/2010/678360/ by S Yashmin (2010)

3. www.isca.in/IJENS/Archive/v5/14/9.ISCA-IRJEvS-2016-011pdf