When talking about the benefits of pyrethrum it doesn’t seem sufficient to simply say that it is a natural product with no environmental residual effects. A reference needs to be made to what the alternatives are so that a comparison can be made.

Consider this information from Wikipedia about the alternative.

Imidacloprid,

As of 1999, imidacloprid was the most widely used insecticide in the world.

• it belongs to a class of chemicals called the neonicotinoids 
• imidacloprid breaks down slowly in water, and thus has the potential to persist in groundwater for extended periods. 
• In soil under aerobic conditions, imidacloprid is persistent with a half-life of the order of 1–3 years
• To … the western honey bee, imidacloprid is one of the most toxic chemicals ever created as an insecticide. 
• Imidacloprid is highly toxic … to aquatic invertebrates

If we assume the worst case for the half life of imidacloprid as being 3 years then this means that 3 years after the paddock was sprayed the insecticide will still be available in the soil at 50% of its original level.

After 6 years this will have halved again to 25% and after 9 years it will have halved again to 12.5%

Therefore…. a decade after it was sprayed on a paddock this insecticide is still active at a rate of approximately 10% of its applied rate.

This is the problem with synthetic insecticides…it is their persistence in the environment, both at the source of their application and into the waterways which they eventually end up.

During the decade plus that imidacloprid is active in this paddock any insect that visits a plant growing in that field will die. Any insect attempting to grow a larvae in the soil of that field will die (eg cicada)  

This product was banned in Europe in 2018 but is still legal in New Zealand. (see a product called Confidor sold in most garden centres across the country)