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Less Droplets, Same Effectiveness on Cockroaches
1 Oct, 2005 By: Dr. William H. Robinson Pest Management ProfessionalWhen spraying surfaces with liquid insecticides, it's common to sweep back and forth to "wet" the surface. The common thought is that the insecticide should completely cover the surface to kill the target pest, but that's not true.
 William Robinson |
Spraying to wetness may be effective, but it's certainly not efficient. Research shows that simply spraying to leave a pattern of droplets (200 times less than wet) provides the same efficacy as spraying to wet.
On surfaces treated to wet, German cockroaches walking on them are killed in about seven minutes, while surfaces treated with 200 times less insecticide than wet will kill cockroaches in about 14 minutes.
 Spraying surfaces to "wet" and applying 200 times less than wet provides effective control of insects contacting the treated surface. |
In the end, the cockroaches are dead within 15 minutes no matter which method you use, and you don't need to coat the surface with liquid to kill them. The droplet approach saves insecticide, money and clean-up time.
DROPLETS
Coarse fan nozzles are usually recommended for spraying indoors because they have a two-foot linear treatment area when held 14 inches from the surface. When used in this fashion, the droplets are large enough that they don't drift off target, whereas fine-fan nozzles produce small droplets that may remain in the air and drift off target.
 This scanning electron micrograph shows segments of a German cockroach foot (tarsus). Note the tarsal pads, and the long setae and spines that help to dislodge insecticide residue on surfaces upon which the cockroach walks. |
One pass of the sprayer wand with a coarse fan nozzle delivers a pattern of droplets that dry to form an effective residue. Research shows that walking on the residue of 33 droplets will knock down susceptible cockroaches.
The lethal dose of insecticide is delivered through the feet, which have large and small spines on the underside and also around the tarsal pads. The small pads on the underside of the feet contact the dried residue during normal walking, and these spines contact and dislodge insecticide residue. The insecticide on the tarsal pads and leg spines enters the body through the mouth during leg-grooming or enters directly through the pads.
KNOCK DOWN
Once through the cuticle and inside the leg, insecticide is carried in the bloodstream to all parts of the body, aided by cockroaches' internal structures.
Blood moves down the leg, and it travels in a channel on the underside once it reaches the feet. Here, the blood picks up any insecticide that has penetrated the tarsal pads.
Then the blood flows to the tip of the foot and is diverted to a dorsal channel on the topside of the feet. Once the insecticide is in the large sections of the leg, it moves into the body where the toxic action occurs. Walking on only a few dozen droplets can produce this sequence of events, so it's overkill to wet the surface with liquid insecticide.
Robinson is president of Urban Pest Control Research and Consulting in Christiansburg, Va. Contact him at wrobinson@questex.com.
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