‘Natural’ Pesticides – Softer on good insects?

There is an increasing trend towards organic and natural products – from food, cosmetics, and even down to the choice in pesticides. It’s not uncommon for people to lean towards or prefer a pesticide that is ‘natural’. After all, an unnatural pesticide will be more harmful, right? However, there’s a discrepancy between perceived safety of ‘natural’ and the reality. For starters, there seems to be no regulation on the word “natural”. As the FDA puts it,

From a food science perspective, it is difficult to define a food product that is ‘natural’ because the food has probably been processed and is no longer the product of the earth. That said, FDA has not developed a definition for use of the term natural or its derivatives. However, the agency has not objected to the use of the term if the food does not contain added color, artificial flavors, or synthetic substances.FDA

So the term ‘natural’ doesn’t mean much, except in Vermont, where they have recently passed a bill (H.112) that prohibits the use of the words “natural”, “naturally made”, “naturally grown”, “all natural” and “other similar descriptors” to describe foods with genetically engineered foods. But still, that definition doesn’t touch on what constitutes ‘natural’ pesticides. Calling something ‘organic’, on the other hand, is regulated by the USDA (see “Guide for Organic Crop Producers” for some quick details) and organic pesticides must be registered by the Environmental Protection Agency (EPA). One of the criteria prohibits the use of synthetic pesticides (“Decision Tree for Classification of Materials as Synthetic or Nonsynthetic“) organic farmers must use pesticides derived from natural sources (i.e. biological pesticides).

The EPA defines three different types of biopesticides:

1. Microbial pesticides (=microorganisms acting as the main mode of action).

2. Plant-incorporated protectants (i.e. inserting genes into a plant that code for protective proteins).

3. Biochemical pesticides (i.e. “naturally occurring substances”) that control pests by non-toxic mechanisms.

The third one is particularly interesting, because it is hard to “determine whether a substance meets the criteria for classification as a biochemical pesticide” (EPA, 2014). There are approximately 200 biochemical pesticides registered, with non-toxic modes of action including growth/developmental changes (plant growth regulators/insect growth regulators), lures/attractants/repellents (semiochemicals), suffocation, desiccation, coatings (irritants & barriers) and induced resistance (Jones, R. S. Biochemical pesticide classification, types, & modes of action 5.1).

Although pesticides with non-toxic modes of action (biochemical pesticides) might be less harmful to humans and mammals, such is definitely not always the case when it comes to beneficial insects. Beneficial insects are those that you want to keep around, because they either eat bad insects or they may be pollinators. Horticultural oils, for example, acting by suffocating an insect is indiscriminate, thereby potentially suffocating any insects that are hit by the insecticide.

The table below shows an example of a study that tried different concentrations of a horticultural oil; a product often considered to be very ‘natural’. The first row, labeled “Untreated check” is our control; nothing was sprayed here, so if the pesticide had no effect, we would expect to have results similar to this row after an application. The column “Rate % v/v” gives the rate of the pesticide sprayed in volume/volume (i.e. 0.5 mL horticultural oil/1000 mL water), and as we go down the column, there is an increase in the concentration of the horticultural oil in the formulation. I blocked out the name of the actual product, because I don’t want to point fingers at just one company here, but the “Treatment/formulation” is all the same horticultural oil product. The point is to look at the affect of horticultural oil on beneficial insects (i.e. ladybeetle larvae and ladybeetle adults). Each column shows the mean number of insects. One of the insects harmed the most by the horticultural oil was ladybeetles; natural predators of aphids, whiteflies and other harmful pests, whereas whiteflies remained unaffected by the horticultural oil. Here is a prime example of how a ‘natural’ product can actually hurt the ‘good’ things and do nothing to the ‘bad’ things. Hmm.. ‘natural’ ain’t always good then!

Efficacy of insecticide at different rates
Stansely P. A. and Conner, J. M. (2006). Management of pests of bell pepper with horticultural mineral oil, 2004. Arthropod Management Tests, AMT31.

The takeaway here is that even though something is ‘natural’ or considered ‘safe’, doesn’t mean that it wont have undesirable effects or will harm bad things only.  That’s why sometimes having a highly specific insecticide that has a toxic mechanism may be considered a more sustainable pest management strategy and less harmful to beneficials than a broad-spectrum non-toxic ‘natural’ spray (i.e. horticultural oils).

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