Natural insecticides, great promises?

Secondary metabolic convergence, a new interpretation

In recent years, plant-based insecticides, often referred to as biopesticides, have returned to the center of international agricultural debate. Their spread has been favored by tightening regulations on synthetic insecticides, the growing demand for low-impact supply chains, zero-residue products, and the need for tools compatible with the principles of
agroecology (Isman, 2020).
In this scenario, biopesticides have often been presented as “natural” alternatives and are perceived as intrinsically safer. However, an in-depth analysis of the scientific literature shows that this natural-equals-safe equation is sometimes scientifically fragile.
The meta-analysis conducted by Turchen et al. (2020) highlights that the problem is not the scarcity of studies on the subject, but the way the research has been structured.
Since World War II, thousands of studies have analyzed the action of plant extracts, essential oils, and secondary metabolites, either individually or in clusters, with insecticidal activity. Furthermore, since the 1990s, scientific production in this specific field has grown exponentially; however, this increase has occurred only quantitatively, that is, it has not been accompanied by an increase in the ecological variables taken into consideration (Turchen et al., 2020).
All the studies considered in the aforementioned meta-analysis (2,500 studies from 1945 to 2019) highlight that the evaluation of the efficacy of plant extracts in various forms derives predominantly from laboratory work and is based almost exclusively on a single parameter: the mortality of the target pest.
Furthermore, it is noteworthy that most studies focus on a limited number of botanical families from which these biopesticides are derived, particularly Meliaceae, Lamiaceae, Asteraceae, and Myrtaceae, generating a high level of experimental repetition and reducing the exploration of the secondary botanical and metabolic biodiversity present in various regions (Isman, 2020).
This approach reflects a classical toxicological paradigm, which, however, is poorly aligned with the actual functioning of agroecosystems, which, as we recall, are complex systems with nonlinear reactions.
In fact, in field conditions and in Mediterranean contexts, characterized by high solar radiation, high temperatures, and rapid degradation of substances, exposure of target insects is discontinuous and sublethal.
In these environmental conditions, data highlight physiological and behavioral changes in the pest, with nonlethal effects.
Numerous studies demonstrate that biopesticides can induce sublethal effects, such as reduced feeding, impaired orientation, altered oviposition, reduced fertility, and changes in biochemical communication in target pests (Desneux et al., 2007; Bartling et al., 2024).
These effects, while not causing immediate death, can equally and profoundly impact population dynamics.
Furthermore, it should be emphasized that sublethal effects are systematically ignored or underestimated in scientific research. This shortcoming is particularly significant when assessing the effects on non-target organisms.
Predators, parasitoids, and pollinators are rarely included in experimental protocols, despite their central role in biological defense systems. It is widely demonstrated that even plant-based insecticides, essential oils, and various extracts can produce negative effects, both lethal and sublethal, on beneficial arthropods, altering their predatory capacity, locomotion, and ecological efficiency (Giunti et al., 2022).
These shortcomings in scientific observation of the phenomenon indicate that the use of natural insecticides as simple “alternative biopesticides” with harmless side effects risks replicating the same conceptual errors made in the testing of synthetic chemical products. Therefore, a paradigm shift is necessary.
A new approach derived from chemical ecology and evolutionary biology can help us: secondary metabolic convergence.
Numerous studies demonstrate that evolutionarily distant plant species, when subjected to similar ecological pressures, can produce identical secondary metabolites or functional groups, or even activate the same metabolic pathways. This phenomenon, known as metabolic evolutionary convergence, is widely documented in higher plants (Pichersky & Lewinsohn, 2011).
At the same time, more recent research has shown that many secondary molecules are shared, or functionally recognized, by both plants and the phytophagous insects (in this case) they control.
In fact, we start from a hypothesis: metabolic or biochemical convergence between plants and insects suggests that these metabolites do not act as general-purpose toxins, but as true biological signals that are not always completely toxic within specific ecological pathways (Beran et al., 2019).
Moreover, if we think about it, in nature, secondary metabolites are not created to kill, but to communicate, modulate, and direct interactions and behaviors. Volatile compounds such as flavonoids, terpenoids and phenylpropanoids, induced by the action of herbivores, play key roles in plant–insect communication and in the regulation of insect modulation and presence (McCormick et al., 2012; Heil, 2014).

Secondary Metabolism → Biochemical Signal → Behavioral Induction → New Equilibrium
In light of these arguments, the effectiveness of natural insecticides can be reinterpreted through a new conceptual framework.
Plant metabolism produces a dynamic set of secondary metabolites, modulated by genetics and epigenetics (environment).
These secondary metabolites become biochemical signals that insects can perceive through multiple pathways: contact, ingestion, or volatilization (Holopainen & Blande, 2013).
Metabolomics, on which secondary metabolic convergence is based, goes beyond the label of the botanical family of origin or the single molecule, but gets directly to the target.
From an almost linguistic perspective, it allows the desired action to be selected based on the coincidence of specific biochemical signatures between the plant and the pest and shared ecological functions between the two organisms. This allows for species-specific action on a target, stimulating effects such as repellency, deterrence, anti-oviposition, and other regulatory behaviors.
This results in a paradigm more suited to a “Specific Signal → Specific Behavior” approach.
Note that these secondary metabolites are widespread in agroecosystems at very low concentrations; nevertheless, they act profoundly, inducing significant and species-specific behavioral responses: feeding deterrence, reduced oviposition, disorientation, and modification of habitat use.
These responses do not cause mortality, but rather reshape the pest’s actual pressure on the crop (Desneux et al., 2007).
When behavior is modified/remodulated without destroying or significantly influencing the food web or the direct metabolic connection between plant and insect, the agroecosystem tends toward a new functional equilibrium, characterized by lower selective pressure, greater stability, and the preservation of ecosystem services (Isman, 2020).
The practical application of this paradigm consists of selecting plant matrices and extracts not based on their “toxic strength,” but on their selectivity or secondary biochemical coincidence between two specific subjects. If one of the two subjects is unable to recognize the molecule (positively or negatively), the mechanism cannot be applied. Therefore, there must be a species-specific biunivocity between the two subjects.
The idea, therefore, is to use metabolic convergence as a sort of laser, capable of ensuring coherence of action and precise targeting.
From a biodistrict perspective, this paradigm could be strategic for several reasons:
– it would enhance local biodiversity and spontaneous plants;
– it would reduce dependence on external inputs;
– it would favor low-concentration approaches and targeted treatments;
– it would strengthen the role of field technicians as facilitators of ecological processes;
– it would integrate defense, fertility, and landscape management;
This approach would enhance territorial matrices and spontaneous biodiversity, reinforcing the principles of the circular economy.
It is therefore not a matter of “choosing just a natural product,” but of choosing a species-specific natural product based on the secondary metabolic convergence present in the area.
All this translates, at the district scale, into a true design of defense as a process based on the secondary metabolic convergence present in a specific area, thus consistent with the climate, crops, biodiversity, and local ecology.
A true ecological transition does not consist of replacing one active ingredient with another, but of changing the logic with which it is chosen within the initial geobotanical characteristics on which the agroecosystem is built.
Natural insecticides, interpreted through secondary metabolic convergence, become agroecological support tools, capable of enabling selective dialogue with and within the system.
If anything, they determine a new ecological structure capable of achieving, by aiming in a straight line, a new ecological balance.
This marks a fundamental shift: from indiscriminate defense to defense as the selective construction of a new biological balance.
It is in this space that agroecology can express its deepest value: not simplifying complexity, but learning to interpret it.
Using molecules already present in an area and functionally recognized as selective between two species reduces the risk of unwanted ecological interference, while simultaneously allowing the use of low doses and regulatory rather than destructive strategies.
In this context, sublethal endpoints do not represent a limitation, but the very core of effectiveness. Selectively reducing the feeding or fertility of a pest is more sustainable than eliminating it completely.
Natural insecticides hold great promise for agroecology, but not because they are “natural.” Their true potential emerges when their ecological selectivity is identified through secondary metabolic convergence.
It is therefore necessary, before indiscriminate use of the natural extract, to identify its specificity of action and species. This identification is easily achieved through the use of public and open-source databases.
Secondary metabolic convergence would overcome the limitations highlighted by current literature by identifying selectivity based on metabolomics, mitigating the difficulty of transferring experiments from the laboratory to the field, avoiding the risk of involving non-target organisms, and reducing pest mortality as the primary effect.
In a future agriculture oriented towards resilience, molecules, even natural ones, are not simple active substances and do not work individually: they are biological information.
Understanding their language is key to transforming pest control from an act of control to a process of rebalancing.

Francesco Di Lorenzo
Agronomist

Essential Bibliography

Bartling, M. T., et al. (2024).
Current insights into sublethal effects of pesticides on insects.
International Journal of Molecular Sciences, 25(11), 6007.
https://doi.org/10.3390/ijms25116007

Beran, F., Köllner, T. G., Gershenzon, J., & Tholl, D. (2019).
Chemical convergence between plants and insects: Biosynthetic origins and functions of commonsecondary metabolites.
New Phytologist, 223(1), 52–67. https://doi.org/10.1111/nph.15718

Desneux, N., Decourtye, A., & Delpuech, J.-M. (2007).
The sublethal effects of pesticides on beneficialarthropods.
Annual Review of Entomology, 52, 81–106. https://doi.org/10.1146/annurev.ento.52.110405.091440

Giunti, G., et al. (2022).
Non-target effects of essential oil-based biopesticidesfor crop protection.
Biological Control, 176, 105071. https://doi.org/10.1016/j.biocontrol.2022.105071

Heil, M. (2014). Herbivore-induced plant volatiles: targets, perception and unanswered questions.
New Phytologist, 204(2), 297–306. https://doi.org/10.1111/nph.12977

Holopainen, J. K., & Blande, J. D. (2013). Where do herbivore-induced plant volatiles go?
Frontiers in Plant Science, 4, 185.
https://doi.org/10.3389/fpls.2013.00185

Isman, M. B. (2020).
Botanical insecticides in the twenty-first century—Fulfilling their promise?
Annual Review of Entomology, 65, 233–249. https://doi.org/10.1146/annurev-ento-011019-025010

McCormick, A. C., Unsicker, S. B., & Gershenzon, J. (2012).
The specificity of herbivore-induced plant volatiles in attracting herbivore enemies.
Trends in Plant Science, 17(5), 303–310. https://doi.org/10.1016/j.tplants.2012.03.012

Pichersky, E., & Lewinsohn, E. (2011). Convergentevolution in plant specialized metabolism.
Annual Review of Plant Biology, 62, 549–566. https://doi.org/10.1146/annurev-arplant-042110-103814

Turchen, L. M., et al. (2020).
Plant-derived insecticides under meta-analyses: status, biases and knowledge gaps.
Insects, 11(8), 532.
https://doi.org/10.3390/insects11080532

Photo source:
– Turchen et al. (2020) Interazione tra le famiglie botaniche e gli organismi bersaglio ottenuti dall’analisi bibliografica di articoli sugli insetticidi botanici (n = 2543);