Key point in one sentence:
Not every spray kills bed bugs — against resistant populations the active ingredient matters far more than the brand or the price, so check the ingredients and the research first.
If the bed bugs in your home are already resistant, spraying more of the same product will not save you. A 2025 Rutgers University study found that a single pyrethroid killed only 0–13% of two resistant strains — and “pyrethroid + neonicotinoid” combo products did no better, also 0–13%. The classes that still work do so in completely different ways — above all the desiccant dusts, which physically dry the insects out and killed 100% of every strain tested, resistant ones included. Stop choosing sprays by brand; choose by chemical class. For background on how resistance develops, see our guide to super bed bugs and insecticide resistance.
Five classes at a glance
The mortality figures below come from the Rutgers 2025 study (Journal of Economic Entomology) — a dry-residue test in which bed bugs contacted a dried insecticide film for five minutes.
| Chemical class | Example ingredients / products | Susceptible strains | Resistant strains | Time to kill | Key point |
|---|---|---|---|---|---|
| Pyrethroids | Permethrin, deltamethrin | 90–100% | 0–13% | Fast | Most common — and the resistance hotspot |
| Pyrethroid + neonicotinoid combos | Transport Mikron (bifenthrin 6% + acetamiprid 5%) | Effective | 0–13% | Fast | “Combo” does not mean it beats resistance |
| Neonicotinoids | Imidacloprid (Prothor SC 200) | Effective | No standalone mortality figure published | Fast | Different target site — pyrethroid resistance does not carry over |
| Pyrroles | Chlorfenapyr (Phantom) | Effective | Still effective — unique mechanism | Slow, 24–48 hours | Bugs stay mobile and spread it; mainly professional use |
| Desiccant dusts | Diatomaceous earth, amorphous silica | 100% | 100% | Slow, 4–6 days | Physical desiccation — there is no resistance to it |
| Isoxazolines (new class) | Isocycloseram | 100% in lab tests | 100% in lab tests | 3–4 days | Laboratory stage only — nothing you can buy yet |
Class by class
Pyrethroids: the default choice — and the first to fail
Pyrethroids attack sodium channels in insect nerves; most household bed bug sprays are built on them. The problem: bed bugs have evolved kdr and super-kdr gene mutations that stop the chemical “key” fitting its “lock”, plus thicker cuticles and stronger detox enzymes. In the Rutgers test, susceptible strains died at 90–100%; the two resistant strains fell to just 0–13%.
Combo products: don’t be fooled by “dual action”
Many people assume a pyrethroid blended with a neonicotinoid must be stronger. The same Rutgers experiment says otherwise — the combo also scored 0–13% against those resistant strains. Resistant populations often combine target-site mutations with metabolic detoxification, which a two-in-one label does not bypass. Combo ≠ resistance solved.
Neonicotinoids: a different target, so kdr resistance doesn’t carry over
Neonicotinoids act on nicotinic acetylcholine receptors — an entirely different system from the pyrethroid sodium channel, which is why the most common pyrethroid resistance mechanisms (kdr, super-kdr) do not affect them. To be straight about the evidence: the Rutgers study published no standalone mortality figure for a pure neonicotinoid, so we won’t invent one. By mechanism and field experience, though, this class remains a sound option for suspected resistant cases (see the Hong Kong section below).
Chlorfenapyr (pyrroles): slow, but a mechanism resistance can’t easily touch
Chlorfenapyr (e.g., Phantom) disrupts the insect’s ability to produce cellular energy — a mechanism unlike any mainstream insecticide. Bugs don’t die immediately; death takes 24–48 hours, and meanwhile they keep moving, which can carry the product back into harbourages and onto other bugs. It stays effective against resistant strains, and is mainly a professional-use product.
Desiccant dusts: the slowest — and the one with no resistance problem
Diatomaceous earth and amorphous silica are not poisons: they damage the waxy coating of the bed bug’s shell so it dehydrates and dies — a purely physical action insects cannot evolve resistance to. In the Rutgers test they killed 100% of every strain. The trade-off is speed: 4–6 days, whereas a deltamethrin dust killed susceptible bugs in about 4 hours. They must be placed where bugs walk and hide.
Isocycloseram: impressive, but you can’t buy it
Isocycloseram belongs to the new isoxazoline class. A Rutgers laboratory study found 100% mortality within 3–4 days, with residues still effective after 30 days. But be clear: this is laboratory-stage research, and there is no consumer product to buy yet. Don’t let online ads borrow the name to sell you something else.
Hong Kong practice: we use Prothor SC 200
Now for field practice in Hong Kong. In our own pest-control work, the main chemical tool is Prothor SC 200 (Ensystex; imidacloprid 200 g/L, a neonicotinoid). The reasons are practical: in our experience it kills fast, and we have not run into resistance problems with it — none of the “spray and spray and nothing dies” situations pyrethroids now produce. The science lines up: imidacloprid targets nicotinic acetylcholine receptors, nothing to do with the sodium channels that kdr and super-kdr mutations defend. This is practitioner field experience, not a laboratory mortality figure; results still depend on the strain, dilution and application, and professional concentrates belong with licensed operators.
How to choose and use: four steps
- Suspect resistance early. If a pyrethroid used correctly, twice, at label rate makes no difference, don’t just spray more — switch class.
- Rotate classes. Never rely on one chemistry forever; rotating between classes, combined with physical methods, is how you avoid selecting for resistance.
- Combine spray with physical control. Spraying alone rarely eradicates bed bugs. Hot washing, steam, vacuuming and sealing harbourages must happen alongside — see our DIY bed bug removal guide for the full process.
- Use products legally and safely. In Hong Kong, use only pesticides registered with the Agriculture, Fisheries and Conservation Department, follow label directions, and leave professional concentrates to licensed companies.
FAQ
Q: I emptied a whole can of spray and the bed bugs are still there. Did I buy the wrong brand? A: Probably not. Most household sprays are pyrethroids; if your bed bug population is resistant, every brand in that class performs about the same — it’s a class problem, not a brand problem.
Q: Is a combo spray always better than a single-ingredient one? A: No — in the Rutgers 2025 test, the combo managed the same 0–13% on resistant strains as a single pyrethroid.
Q: If desiccant dusts kill 100% of resistant strains, why not use only dust? A: Because they’re slow (4–6 days) and only work where bugs actually walk through them. In practice, dusts guard the cracks while other methods do the main work.
Q: Do neonicotinoids never face resistance? A: Don’t count on it. They simply aren’t affected by kdr-type pyrethroid mechanisms. Any insecticide used alone for long enough can select for resistance in theory — which is exactly why rotation matters.
Q: I suspect resistance. DIY or call a professional? A: A small, early infestation is worth a proper DIY attempt. If spraying makes no difference or the problem spreads, you’re likely facing a resistant strain or deep harbourages — a job for a licensed company with professional products and an integrated approach.
Sources
- Rutgers University study, Journal of Economic Entomology (2025), DOI: 10.1093/jee/toaf244 — five-minute dry-residue contact test: pyrethroids and combo products at 0–13% mortality on resistant strains; desiccant dusts at 100% across all strains; plus the isocycloseram laboratory data.
- Prothor SC 200 product information (Ensystex; active ingredient imidacloprid 200 g/L).
- The “Hong Kong practice” section reflects this site’s pest-control field experience, not laboratory data.
(Written 20 October 2026; draft, not published.)