Organic Pesticides and their Impacts on Beneficial Insects
The topic of examining the effects of pesticides on bees and beneficial insects has been an overarching theme these past few months, from investigations on off-target effects on Apis mellifera to discussions on the sublethal effects of herbicides and fungicides on bees. Natural pest control measures like integrated pest management practices were touched upon, and this article will elaborate on organic products as alternatives to the conventional pesticides available for purchase.
Do “naturally occurring” chemicals make organic pesticides safer for use? After all, the pyrethrin pesticides (eg Raid Flying Insect Killer, Kicker Insecticide) are made from the seed casings of certain chrysanthemum flower species, and can harm bees.
These products can be marketed as natural, “organic” pesticides, and yet still impact pollinator. Be wary of those claiming that organic pesticides should be deemed harmless because they are derived from natural sources or do not become systemic in plant tissues. Some of these pesticides still display toxicity toxic to bees on contact or ingestion, handle with care.
Types of Natural Pesticides
A combination of The Science™ and knowledge that predates science gives optionality on natural pesticides for use in the home garden. Here I define “natural” as derived from plants, bacteria, fungi or minerals, not synthesised in a laboratory from natural precursors (Pyrethrins started out as a natural pesticide, and then the synthetic pyrethroids arose). Note that “non-systemic” pesticides can leave residues on flower surfaces, nectar or pollen where bees can be exposed. The “non-systemic” label is there to indicate that the active ingredient isn’t taken up into tissues, not that it can’t come into contact or be ingested by bees.
Organic Pesticides from Plants
Botanical insecticides are secondary metabolites produced by plants for defence against pests and disease. Like the pyrethrum from Chrysanthemum flowers mentioned above, there is also:
- Nicotine, from tobacco plants – a neurotoxin that is no longer in use due to extensive non-target toxicity. Don’t forget its importance in Big Tobacco’s bottom line!
- Neem oil, extracted from the seeds of the neem tree – neem oil contains several compounds with insecticidal activities, with the most well known being azadirachtin, which induces insect growth disruptions.
- Rotenone, a compound extracted from the roots of legumes – effective against insects and fish on account of mitochondrial disruptions. However due to this aquatic toxicity, it is restricted in use.
- Quassia, a bark extract from Quassia amara trees – frequently used as an organic garden pesticide. In insects, it interferes with the digestive system and deters feeding, leading to death by starvation (Sama et al., 2014). This same digestive disruption is how it found its way to being a health food for gastrointestinal conditions.
- Sabadilla, a lily seed extract – is a potent neurotoxic insecticide due to the presence of various alkaloids like cevadine and veratridine (Duke et al., 2010). When insects come into contact with sabadilla, the alkaloids disrupt the sodium channels within their neurons, leading to paralysis and death.
- Honourable mention to capsaicin, the powder that gives chili peppers their heat. It is toxic to insects, as it the molecules disrupt nerve functions, but in humans, capsaicin leads to a mild heat irritation that many find pleasant!
In the context of bees, both wild and honey bees can be impacted by these natural pesticides (see Xavier et al 2015 for a review of sublethality from botanical insecticides). Such compounds are more readily broken down in the environment, and unlike commercial (& systemic) pesticides, the individual foraging bee must make physical contact with the application for it to have an effect. When viewed from this perspective, these natural pesticides are safer for residential applications and less likely to cause sublethal effects compared to synthetic pesticides like neonicotinoids.
Organic Pesticides from Microorganisms
There are organic pesticides extracted from microorganisms such as bacteria and fungi. The posterchild of this group is Bacillus thuringiensis, Bt, which gave rise to Bt cotton, a transgenic species of cotton that produces its own Bt toxins against cotton pests like caterpillars, beetles, and mosquito larvae.
In the field, Bacillus thuringiensis can be purchased as a pesticide spray or in granules for use in organic gardens. When the target insect ingests the bacterium, it travels to its gut, where its alkaline digestive environment (pHs 9.0-10.5) leads to proliferation and subsequent release of Bt toxins, which damages the gut lining, leading to starvation and death. Bt toxicity is specific, with various strains available to target the pest of interest.
B. thuringiensis is not toxic to bees alone, and experiments applying pure formulations of the Bt aizawai subspecies found the median lethal dose (LD50) greater than 100 μg/bee, thus considered practically nontoxic. Any harm to pollinators was found to be mediated by the synthetic chemicals used as adjuvants in the commercial pesticide formulation (Steinigeweg et al 2021). Further, BT aizawai has also been approved for use in controlling wax moths by spraying formulations onto unused frames.
Spinosad and Honey Bee Toxicity
Spinosad is derived from the bacterium Saccharopolyspora spinosa. Like B. thuringiensis, S. spinosa produces toxic secondary metabolites (spinosyns), which activate the acetylcholine receptors in the insect nervous system, causing paralysis and death (Sparks et al., 2001). Spinosad has shown high acute toxicity to honey bees under laboratory conditions and in the field, when formulations were still wet. Dried spinosad residues were not found to be harmful to bees and other pollinators, making it a prominent insecticide in organic and sustainable agriculture operations.
A 2019 study by Christen & colleagues found that honey bees exposed to spinosad exhibited altered expression of genes involved neuronal signalling, immune regulation, energy metabolism and hormone regulation. These effects were found fluctuates with the season, with more intensive changes in the spring than summer, correlating with the increases in pesticide application. The authors found several other changes on gene expression, and concluded that despite the low toxicity, these changes in transcription profiles will have sublethal effects on bees over the long term.
Using Fungi as Organic Pesticides
The fungus Beauveria bassiana controls pests by releasing spore that attach to the insect exoskeleton, penetrating the cuticle and killing the insect by growing within it (Wang et al., 2021). The toxic secondary metabolites form a white mould over the cadaver and is known as white muscardine disease., used for controlling termites, white flies, and aphids, amongst others. These spores are sprayed onto affected crops, and like B. thuringiensis, show differing effectiveness based on the fungal strain.
Another fungal species are Metarhizium. Along with B. bassiana, these biochemical pesticides grow in the insect cuticle, leading to the appearance of green muscardine disease (Shah et al., 2022). Most insects that dwell near soils have evolved protective adaptations to these fungal infections. It is only when the species is applied on pests that have not had previous exposure to these fungi that they become effective. Several species and strains have been used as biological pesticides for control of termites and thrips, with novel applications include Metarhizium-coated mosquito nets to combat malaria (Lovett et al., 2019).
grows inside the insect and leaves the brain intact, allowing it to manipulate the insect’s behaviour. It then directs the insect to climb to a high point where it dies and fungus grows from its head, to produce the fruiting bodies that will spread stores and infect new hosts. It’s also edible! Image credits: Facebook group Beekeeping Basics
Organic Pesticides from Minerals and Other Sources
As far as organic pesticides go, these are perhaps the least harmful to bees as they do not exhibit systemic effects following application. For individual bee to become affected, it must come into physical contact with the formulation. Yours truly uses diatomaceous earth as a natural insecticide inside honey bee hives for the control of small hive beetles; these fossilised diatoms are sharp enough to pierce through the beetle cuticle, leading to death by dehydration.
Minerals have been used as organic pesticides for a long time and range from sulfur for the control of plant pathogens to boron and for cockroach control. These substances are mined or derived from nature (eg diatomaceous earth) and have found their way into many homemade pesticides. Some, like copper compounds, can accumulate in the soil and are used sparingly, while others like lime sulfur, can be freely sprayed onto leaves (not in hot weather though!).
Viruses and Nematodes as Organic Pesticides
The “other sources” listed above refers to viruses and nematodes. Organic pesticides derived from viruses have been approved for agricultural use and are highly specific to their target organism. Two approved viruses are:
- Baculoviruses, for the control of caterpillars. Infected caterpillars stop eating and eventually starve to death, which is results in rapid viral proliferation that spreads to other organisms upon release from the cadaver.
- Granulovirsuses, which have been used in the control of coddling moths in North American pear and apple trees by preventing moth larvae from burrowing into the fruit, thus disrupting their development.
Nematodes refer to roundworms in the genera Steinernema and Heterorhabditis, and have been used to control soil-dwelling pests such as beetles & their larvae. Juvenile nematodes invade the body cavity of insects and release symbiotic bacteria Photorhabdus and Xenorhabdus (found inside Heterorhabditis and Steinernema respectively), who kill the target within 24 hours. This insect cadaver is now a food source for 2-3 generations of nematodes, who spread the bacteria to new nematode juveniles, thus restarting the life cycle (Kiontke & Fitch, 2013).
Pesticides and Organic Honey
As previously mentioned, the presence of pesticides in honey is almost guaranteed where humans work with plants. Organic agriculture attempts to address these concerns by producing foods free from pesticides, however cross-contamination can still occurs from aerial drift or irrigation, making food regulations difficult. Given that honey bees forage up to 10 kilometers from their hive, it is very likely that urban beekeepers would be unable to use the “organic honey” label as they cannot control what chemicals are used in their neighbour’s garden.
Guidelines around organic certification have been developed, and vary between regions:
- In Australia, it is summarised as a minimal chemical intervention, using pure hive products and placing hives at least 5 kilometers away from pollination sources to avoid contamination.
- The US Department of Agriculture’s guidelines are similar, requiring records for land management, hive management, honey harvests, packaging and labeling usage.
- Section 7.1 of the Canadian Organic Standards provides further guidelines for feeding and maintaining organic honey bees, such as only allowing supplemental feeding during natural forage shortages, and pesticide treatments (eg for Varroa mites) can only be applied if the if the resulting honey supers are replaced with organic wax at the end of the treatment period
“Organic” does not mean “pesticide-free”. Some of the biological and botanical pesticides listed above are approved in agriculture and this extends to beekeeping – so long as they meet local organic standards and are used in accordance with the label directions, the produce can be certified organic (that doesn’t mean these pesticides that don’t kill bees).
Avoiding Synthetic Pesticides
Trace concentrations of synthetic pesticides in food products are a health concern. Like the sublethal effects pesticides have on bees, we too could be slowly accumulating low-level pesticide concentrations that could manifest in sublethal conditions down the line. Research into sublethal effects on human health is sparse and focuses on the big picture, high-dose consumption (or poisoning) of pesticides (Ferreria et al., 2022), or laboratory experiments on human cells (many references, such as Liboudo et al., 2014). Often repeated in this series is to assume the precautionary principle: all pesticides are harmful and can lead to lethal or sublethal effects over time.
Whether one wishes to go down the path of homegrown and organic foods, or decide that life is too short to panic about food, that is up to them. the alternative pest management strategies presented here aim to educate and inform gardeners and consumers about what goes into their honey, and by extension, food supply.
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