1. Summary
"Bug repellent" isn't one single thing. Repellents you put on your skin mostly work by making insects stop approaching, landing and biting. Insecticides sprayed into the air or onto surfaces act on the nervous system and can cause overexcitement, loss of coordination and paralysis. Long-lasting spider treatments leave a chemical residue on a surface, so every time a spider or the insects it feeds on touch it, the residue repels or kills them.
2. The question: what does DEET actually do?
DEET (N,N-diethyl-3-methylbenzamide) is the best-known active ingredient in skin repellents. It doesn't just mask the smell of humans. It interferes with a mosquito's behavior through several routes at once: smell, taste and touch. In yellow fever mosquitoes (Aedes aegypti), covering the tips of the legs (the tarsi) to block their chemical sensing made them much worse at avoiding DEET-treated skin. In other words, a mosquito can land, detect DEET with its feet, and give up on biting. [1]
For a human, it's a bit like walking up to food, getting hit by a harsh chemical smell, and then touching a floor that sends back an awful bitter signal. But insects sense the world differently than we do, so we can't say they experience it the same way.
3. The evidence: how do pyrethroids scramble nerve signals?
Pyrethroids are used in household insecticides and in some spider products. Their main target is the voltage-gated sodium channel. Normally this channel opens briefly to produce a nerve impulse and then closes right away. Pyrethroids push it toward staying open, which leads to repeated firing and a lasting electrical imbalance in the nerve cell. At a low dose, you see restless movement and attempts to flee. At a high dose, coordination falls apart and the insect goes into knockdown, paralysis and death. [2]
The closest human comparison is the numbing tingle of sansho (Japanese pepper), or pins and needles on your skin, as if your nerve switches were being mashed on their own. But product concentration, species and the way the insect is exposed all differ, so you can't translate an insect's state into a human sensation one to one.
4. How it works: why can't insects avoid spider webs?
It's not that insects can't see webs at all. Bees and wasps, for example, can learn what webs look like and get better at avoiding them. There are still several reasons collisions keep happening.
- The silk is thin, semi-transparent and low in contrast against the background. Even in bright light, a patterned background makes it harder to see. [3]
- Given how fast insects fly, the distance at which they spot a web is short, so by the time they notice it, it's too late to turn.
- Webs are built where insects travel: near lights, in gaps between plants, around feeding spots.
- Being too cautious has a cost. The insect might not reach food, mates or places to lay eggs.
- In lab conditions, when an electrically charged insect approaches, the silk can bend toward it, which raises the chance of contact. [4]
5. A common misconception: if the caught ones die, can't they learn?
If an insect is eaten the first time, it obviously can't carry what it learned into the next encounter. That part is true. But not every insect dies on the spot. Those that spot the web and avoid it, or touch it and escape, can learn. In experiments, bees and wasps got better at avoiding webs based on their color and pattern, and ants that had escaped a web next time preferentially cut the sticky capture threads and got away faster. [5][6]
Natural selection across generations also plays a part. Traits that help with escape, such as scales, hairs, body-surface structure and flight ability, tend to be passed on. Even so, a web doesn't need to catch every insect. It's a game of chance: it works as long as it catches enough prey to pay for the cost of building and maintaining it.
6. In practice: what makes long-lasting spider control work?
In shared areas of buildings, for example, treatment often goes beyond removing webs. It can involve applying a long-lasting product to ceiling corners, around lights, on walls and into crevices. The effect comes mainly from the surface irritating or killing whatever touches it, and from fewer prey insects around for the spiders to eat. Spiders don't hear a warning from other spiders. They simply suffer a chemical penalty every time they come to that spot.
That said, you can't be sure whether the product actually used was a pyrethroid, a mainly repellent type, or one with a coating agent unless you check the work order or the ingredient list. Even when the active ingredients are similar, store-bought products and professional formulations differ in concentration, form, how long they last and how much area they cover.
DEET for skin and insecticides for building surfaces are meant for different uses. On your body, use only products approved and labeled for human use, and follow the label: concentration, age limits, how often to reapply, and keeping it away from your eyes and not breathing it in. In Japan, standards have been set for the protection time that human-use repellent labels may claim, depending on concentration. [7][8]
7. Conclusion
Repellents use sensory input to stop insects from approaching, touching and biting, while insecticides disrupt the nerve signals themselves. Insects can learn to recognize and avoid spider webs, but they still can't avoid them completely, because of limits in perception, reaction time, the benefits and costs of their behavior, and the physical performance of the trap. Webs work not because insects can't learn, but because even after learning, some failures remain a matter of chance.
References (8)
- Dennis EJ, et al. “Aedes aegypti mosquitoes use their legs to sense DEET on contact.” Current Biology (2019). PMCID: PMC6504582 pmc.ncbi.nlm.nih.gov
- Silver KS, Du Y, Nomura Y, Oliveira EE, Salgado VL, Zhorov BS, Dong K. “Voltage-Gated Sodium Channels as Insecticide Targets.” Advances in Insect Physiology. PMCID: PMC6005695. PMID: 29928068 pmc.ncbi.nlm.nih.gov
- Craig CL. “Effects of background pattern on insect perception of webs spun by orb-weaving spiders.” Animal Behaviour (1990). DOI: 10.1016/S0003-3472(05)80733-X. )80733-X doi.org
- Ortega-Jimenez VM, Dudley R. “Spiderweb deformation induced by electrostatically charged insects.” Scientific Reports (2013). PMCID: PMC3701317 pmc.ncbi.nlm.nih.gov
- Craig CL. “Limits to learning: effects of predator pattern and colour on perception and avoidance-learning by prey.” Animal Behaviour (1994). DOI: 10.1006/anbe.1994.1147 doi.org
- Gonzaga MO, et al. “The role of learning in risk-avoidance strategies during spider-ant interactions.” Animal Cognition (2013). PMID: 23771493 pubmed.ncbi.nlm.nih.gov
- 厚生労働省. 「人体に直接使用される忌避剤の忌避効果持続時間の表示について」令和6年12月11日 mhlw.go.jp
- 厚生労働省. 「防除用医薬品及び防除用医薬部外品の製造販売承認申請に係る手続きについて」平成28年6月15日 mhlw.go.jp
