Onslaught Fastcap Spider & Scorpion Insecticide — 16 oz
Specifically formulated for scorpions & spiders
One of the only insecticides specifically marketed and labeled for bark scorpion, black widow, and brown recluse control. The esfenvalerate + prallethrin dual active formula is engineered for hard-to-kill arachnids that standard pyrethroids often miss.
Pesticide — read label before use
MGK Onslaught Fastcap is a professional-grade insecticide concentrate. Always read and follow label directions. Keep people and pets off treated areas until dry. Avoid application near water bodies, storm drains, and pollinator habitats.
Why we recommend this
For Arizona homeowners, scorpions are the number one pest concern — and Onslaught Fastcap is the product we reach for when standard pyrethroids aren't cutting it. The dual active esfenvalerate + prallethrin formula hits scorpions and spiders harder than single-active products, and the micro-encapsulation keeps it working on stucco and block walls for months. If scorpions are your primary concern, this is the product to buy.
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Key Features
Specifically Formulated for Scorpions & Spiders
MGK Onslaught Fastcap is one of the few insecticides specifically marketed and labeled for scorpion and spider control — including bark scorpions, black widows, and brown recluse spiders. The dual active formula is engineered for hard-to-kill arachnids.
Dual Active: Esfenvalerate + Prallethrin
The combination of esfenvalerate (a potent pyrethroid) and prallethrin (a fast-acting knockdown agent) delivers both immediate kill and long-lasting residual — hitting pests from two different modes of action.
Kills 200+ Pest Species
Beyond scorpions and spiders, Onslaught Fastcap is labeled for over 200 pest species including ants, cockroaches, fleas, ticks, wasps, mosquitoes, and virtually every common household and structural pest.
Micro-Encapsulated Formula
The ME formulation encapsulates active ingredients in microscopic capsules that adhere to surfaces and pest bodies — releasing over time for extended residual activity up to 90 days.
Fast Knockdown + Long Residual
Prallethrin delivers rapid knockdown on contact while esfenvalerate provides the long-lasting residual barrier — two complementary actions in a single application.
MGK Professional Pedigree
MGK (McLaughlin Gormley King) has been a leader in professional pest control chemistry for over 100 years. Onslaught is one of their flagship professional products, trusted by pest control operators nationwide.
Product Specifications
Customer Reviews
Rick A.
The best scorpion product I have ever used
I've lived in Arizona for 20 years and tried every scorpion product on the market. MGK Onslaught Fastcap is in a different league. Sprayed the perimeter and garage in March — it's now late June and I haven't seen a single scorpion inside. The dual active formula hits harder than standard pyrethroids and the micro-encapsulation keeps it working for months. This is the one.
Scottsdale Homeowner
Finally eliminated black widows in the garage
Had a persistent black widow problem in the garage that nothing else could solve. Two treatments with Onslaught Fastcap — one week apart — and the problem was completely gone. The esfenvalerate + prallethrin combination works differently than standard bifenthrin or cypermethrin products. Highly recommend for anyone dealing with spiders.
Pest Control Professional
Go-to product for scorpion accounts
Onslaught Fastcap is my first choice for any account with scorpion pressure. The dual active formula — esfenvalerate plus prallethrin — performs better on scorpions than any single-active pyrethroid I've used. The micro-encapsulation gives it excellent residual on stucco and block walls. MGK makes excellent products and this is one of their best.
Dual active scorpion & spider formula. 200+ pest species.
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