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Open access journal: http://periodicos.uefs.br/ojs/index.php/sociobiology
ISSN: 0361-6525
DOI: 10.13102/sociobiology.v68i1.5813
Sociobiology 68(2): e5813 (June, 2021)
Introduction
Chemophobia or chemonoia, meaning a fear of
“chemicals”, combined with the “appeal to nature” fallacy, or
the false idea that a “natural” chemical is inherently safer than
a synthetic chemical, are signicant drivers of the growing
demand for “natural” and food-based alternatives to products
such as medicine, cosmetics, and pesticides (Francl, 2013;
Shelomi, 2020). While non-chemical therapies and pesticides
certainly have applications in medicine and integrated pest
management respectively, the demand for “natural” chemicals
and rejection of anything seen as “articial” can lead to people
rejecting safe and effective products in favor of alternatives
lacking in safety or proven effectiveness but often higher in
price, and thus suffering needlessly (Johnson et al., 2017). Using
chemophobia to market a product that is not cost-effective is
Abstract
Sociobiology
An international journal on social insects
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itself a negative, as it involves proting from logical fallacies
and misinformation, or at worst committing fraud by making
false statements. Insisting that all claims be supported by
evidence is a solid ethical position, and one can apply the
scientic method to conrm or reject “hypotheses” promoted
by the natural products industry. This “mythbusting” (Zavrel,
2016) can expose ineffective remedies as unethical placebos,
but also can reveal genuine effects of compounds that can
be further investigated, as has happened for several safe and
effective pharmaceuticals derived from plant-based chemical
remedies. A good example is the mosquito repellent para-
Menthane-3,8-diol (PMD), derived from lemon eucalyptus
(Corymbia citriodora (Hook.) ssp. citriodora) and endorsed
by the Centers for Disease Control and Prevention (CDC) as
similarly effective as DEET (CDC, 2019). [It is worth noting
that “natural” oil of lemon eucalyptus is not an effective
M Shelomi, BJ Qiu, LT Huang – A case against myths of plant extracts against ants that are pests
repellent, and that PMD is considered less safe than DEET,
having more documented side effects and stronger restrictions
on its use (CDC, 2019; Shelomi, 2020).]
The difference between science and pseudoscience lies
in the rigor and replicability of the relevant work. Demonstrating
PMD’s effectiveness required years of research from multiple
groups around the world using appropriate, standardized
repellency testing protocols (Carroll & Loye, 2006).
Unfortunately, too frequently one sees papers published in
legitimate or predatory scientic journals concluding that a
product is “very promising,” but with methodological aws
such as impractically high concentrations or biologically
irrelevant experiment designs that render the results
meaningless. Individuals involved in promoting a certain
product (or denigrating a competitor) can thus misuse scientic
publications to provide a veneer of academic respectability to
what is otherwise pseudoscience (Weigmann, 2018). In pest
management this problem manifests as scientists taking any
plant or food readily available to them and testing its effects
against a pest, often in eld-unrealistic concentrations with
low sample sizes and conned laboratory experiments, and
concluding that this “home remedy” formulation is useful,
without any safety testing, any effort to identify the compounds
responsible for the effect, and how to scale it in a cost-effective
implementation. Examples include a study on 41 essential
oils that claimed eight of them were 100% effective, but only
following “a peculiar formulation to x them on the human
skin” (Amer & Mehlhorn, 2006); and a study that claimed a
common seaweed kills mosquitoes, but only when mixed with
a lethal dose of insecticide (Prasanna Kumar et al., 2012). Such
research is abundant yet unhelpful and rarely leads to a cost-
effective product, meaning a product that is safe, effective, and
long-lasting to the point that it is worth using.
The target of study in this paper is the popular myth that
cucumber (Cucumis sativus L.) repels ants. A quick Internet
search reveals over a million hits promoting cucumber as a
“natural” ant repellent. Methods of control include leaving slices
or peels of cucumber anywhere in the home one wants ants to
avoid, using an extract of cucumber in water or another “natural”
chemical solvent, or purchasing an expensive cucumber-based
“natural” product. The same search also reveals YouTube videos
of ants devouring cucumbers without trouble, so clearly ants are
not completely put off by cucumber. Some online sources state
“bitter cucumber” is a better repellent: we could not identify
what plant “bitter cucumber” could exactly relate to, but suspect
it refers to a Cucurbitaceae plant, bitter melon (Momordica
charantia L.)
Like most such claims, the scientic evidence for
cucumber as repellent is scant, though the possibility that
certain Cucurbitaceae contain a compound that, at a high
concentration or in a puried form, repels ants is non-zero. A
report from 1982 testing an “old wives’ tale” that cucumbers
repel cockroaches found that, while whole cucumbers did
nothing, sliced cucumber repelled roaches 80% of the time,
and the active ingredient, trans-2-nonenal, repels roaches
100% of the time (Maugh, 1982). The same researchers
identied two more chemicals inside cucumbers, (E,Z)-2,6-
nonadien-1-al and (E)-2-nonen-1-al, that repel cockroaches
(Scriven & Meloan, 1984). The active moiety of these
molecules can be applied to make even more potent synthetic
compounds, like diisopropyl ether and 5,5-dimethyl-3-ene-
butyrolactone, that are much more effective repellents. The
author noted, however, that these compounds are all highly
volatile (Maugh, 1982): it is likely that cucumber’s repellency
effect wears off quickly, at which point it becomes a slice
of rotting food that would only serve to attract more pests.
The only evidence of cucumbers explicitly repelling ants in
the “scientic” literature are two poor-quality studies from
2013 and 2014 by the same researcher published in predatory
journals, which we are ethically dis-inclined to cite in order to
combat the scourge of predatory publishing (Clark & Smith,
2015; Kurt, 2018).
In the spirit of mythbusting, we thus tested the hypotheses
that cucumber and bitter melon can function as gustatory and/
or olfactory repellents for applicable solutions against ant
infestations.
Material and Methods
Our methods are all derived from published bioassay
literature, albeit of varying quality and relevance to ants, yet
nonetheless with sufcient citations to justify publication.
Whole, raw cucumbers and white bitter melon were purchased
from a grocery store in Taipei, washed thoroughly, and the
peel grated off. To make aqueous solutions, 20 g of either
grated peel were added to 100 mL of reverse-osmosis puried
water and extracted for 24 hours at 4°C. These extracts were
sterilized of any microbes by ltering through Millex® GP
lter units with 0.22 µm pore-size Millipore Express® PES
membranes. To produce other extracts, 20 g of peel were
sequentially extracted in 100 mL each of analytical grade
hexane, isopropanol, and methanol for 24 hours each at
-20°C. Extracts were centrifuged to eliminate solid particles.
Ants were trapped from around the National Taiwan
University Entomology Museum building using a bait
of canned tuna, fructose syrup, and rolled oats. The ants
were identied morphologically as the invasive Pheidole
megacephala (Fabricius) (Wetterer, 2007). Worker ants were
collected into a 50 mL centrifuge tube just before use. Each
individual ant was used once, then killed by freezing. Only
the minor worker ants were used. Major worker ants (soldiers)
were not used.
Experiments were performed in 55 mm diameter
plastic petri dishes, the sides of which had been painted
with Fluon® to keep the ants from escaping. The methods
are modied from those used to test responses to chemicals
in Drosophila (Monte et al., 1989). To test for gustatory
repellency, a circle of cardstock that can t inside the dish is
Sociobiology 68(2): e5813 (June, 2021)
cut in half. One half is dipped quickly in the extract, the other
as a control dipped in the solvent. The two halves are placed
in the dish and allowed to air dry, and twenty ants placed in
the center of the dish with a paintbrush. To control for visual
cues, a box is placed over the petri dishes to block out the
light. After 15 minutes, the box was lifted and the number of
ants standing on the control or extract paper was recorded. For
each plant-solvent combination, a total of 10 replicates was
run. The samples with the most signicant gustatory response
were tested for olfactory response. In those tests, each dish
contained two hole-punches of cardstock placed equidistant
from either end of the petri dish, one wetted with 20 µL of
extract and the other with the control solvent. Twenty ants
were released in the center, and a box used to block visual
cues. After 15 minutes, the number of ants in each half of the
arena was recorded.
For statistical analysis, a response index (RI) (Monte
et al., 1989; Amer & Mehlhorn, 2006) was calculated from
the number of ants on the control (C) and extract (E) side
using this equation: RI=(E-C)/(E+C). The mean RI over
all replicates was recorded as the strength of the effect:
a strongly attractant substance has an RI of 1, a strongly
repellent substance has an RI of -1, and a completely neutral
substance has an RI of 0. The number of insects on the E
and C sides over all replicates for each solvent was compared
using a two-tailed, two-sample, paired t-test, which estimates
the statistical signicance of the repellency (ie: how often or
how reliably a repellent effect can be observed). Two-tailed
tests are more conservative but allow for the possibility of an
attractant effect. The same test was also used to compare the
effects of cucumber to bitter melon with the same solvent.
Results
The results are summarized in Table 1. T-tests
comparing the results of gustatory tests for cucumber and
bitter melon in each of the four extracts showed no signicant
difference in their effect (p > 0.1). The extracts in water and in
hexane had weak (-0.4 < RI < -0.2) but signicant (p < 0.01)
repellent effects, and extracts in methanol had very weak
(-0.2 < RI < -0.1) but signicant (p < 0.01) repellent effects.
Extracts in isopropanol showed no effect (0 < RI < 0.2,
p>0.1). For water and hexane only, we performed olfactory
testing. Bitter melon extracts and cucumber in hexane had
no effect (p > 0.05). Only aqueous extract of cucumber had
a statistically signicant effect (p < 0.001), with an RI of
-0.544, meaning on average 77% of ants were found on the
side of the petri dish with the control water disk, compared to
the 50% expected in a negative control
Discussion
The results suggest that cucumber may indeed repel
some ants, slightly, sometimes. More accurately, the results
show that a puried extract of Taiwanese C. sativus peel in
dihydrogen monoxide (“water”) repelled a slim majority
of Pheidole megacephala workers harvested from a single
colony from one half of a conned container to another, with
an effect lasting at least 15 minutes. We found no signicant
differences between the gustatory repellency of bitter melon
and cucumber, and no olfactory repellency for bitter melon at
all: whatever the online sources regarding “bitter cucumber” are
talking about, it is not Momordica charantia.
The results suggest cucumber is not a particularly
powerful way to repel ants, as some ants were always present
on or near cucumber extract disks or papers. We also cannot
tell from the data how long the effect lasts, how far the
effect spreads, or what concentration of volatiles is needed
for maximum effect. Isolating the compound[s] responsible
may provide interesting results: it is unlikely to be trans-
2-nonenol, (E)-2-nonen-1-al, or (E, Z)-2, 6-nonadien-1-al
(Scriven & Meloan, 1984), as those are insoluble in water.
That said, because the observed effect is still weak, adding
such an analytical chemistry component to this study would
serve mostly to make it appear more publishable and appease
chemistry-minded reviewers.
Fruit Solvent Gustatory Olfactory
Mean RI p-value Result Mean RI p-value Result
Cucumber
Water -0.284 0.007 weak repellent -0.544 <0.001 half repellent
Hexane -0.400 0.019 weak repellent 0.205 0.194 no effect
Isopropanol 0.207 0.421 no effect
Methanol -0.165 0.014 very weak repellent
Bitter Melon
Water -0.356 0.001 weak repellent -0.21 0.100 no effect
Hexane -0.342 0.003 weak repellent -0.156 0.221 no effect
Isopropanol 0.003 0.501 no effect
Methanol -0.172 0.023 very weak repellent
Data is based on 10 replicates for each assay with 20 ants per replicate. No olfactory experiments were done for isopropanol or methanol extracts. p-values are
based on a two-tailed, two-sample t-test. RI = Response Index.
Table 1. Results of Gustatory and Olfactory Repellency Tests of Cucumber and Bitter Melon Extracts on Pheidole megacephala (Fabricius).
M Shelomi, BJ Qiu, LT Huang – A case against myths of plant extracts against ants that are pests
To summarize, while this research did nd “statistically
signicant” effects of cucumber peel extract on ants, it does
not at all suggest cucumber is a good or even promising
repellent. The extract did not approach 100% repellency,
even after only 15 minutes of time, while typically repellents
are measured in terms of hours of absolute repellency (RI of
-1.0). Cucumber extract cannot be considered a cost-effective
repellent, both because of its low efcacy and because
cucumber provides much higher value as a food. Cucumber-
based products marketed as “natural” repellents [and likely
priced accordingly] are almost certainly a waste of money,
unless they have been adulterated with actual repellents.
Fresh cucumber as recommended by the Internet would
likely be even less effective: the bulk of the cucumber is not
particularly aromatic but is rich in nutrients that worker ants
would eagerly take back to their colonies. Spreading ant food
around places where ants congregate does not seem like an
effective strategy for ant management. There are better uses
for cucumbers and better solutions for ant control.
Indeed, the very idea of an “ant repellent,” natural or
otherwise, was misguided from the start: it is a marketing
gimmick for “natural” product pushers, but was never a cost-
effective pest management tool. Repellents are valuable for
temporary personal protection against pests that cannot be
easily eradicated, such as against mosquitoes when hiking in a
natural forest. However, pests that can infest households or in
sensitive eld setups (crops, recreational areas, pastures, etc.),
repellency is simply impractical. Repellents eventually wear
off, and then the pests will return. For cost-effective control
of household insects such as ants or cockroaches, the simple
preventive action of keeping potential food items out of
access can prevent or limit infestation levels, and insecticides
that have demonstrated their efcacy and sustainability can be
used thoughtfully to mitigate pest problems. One of the most
common solutions used for ant control at home is a mixture
of borates (borax or boric acid) with bait such as sugar, which
the workers will take back to share with the colony, causing
signicant population reduction, while using a extremely
small quantity of active ingredient. Unlike cucumber, borates
are both safe and effective, with plenty of peer-reviewed and
rigorous publications supporting their use (Klotz et al., 1998;
Gore & Schal, 2004), with the marketing bonus of also being
“natural,” for anyone who still values that term.
Acknowledgements
Special thanks to Chi-Man Leong for ant identication,
anonymous reviewers and the editors for their suggestions
and revision.
Authors’ Contributions
MS: conceptualization, methodology, writing and revision
BJQ and LTH: methodology, investigation.
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