Gel Baits

Since their introduction several years ago, gel baits have provided the professional pest control operator with an effective means of controlling cockroaches. Once reserved for sensitive areas, baits in the form of gels or stations are now a regular part of most cockroach control programs and are used throughout accounts. The advantages of soft, gel baits are that they can be easily dispensed in varying amounts and they can be placed at specific locations where they will be encountered by cockroaches searching for food. Gel baits are perhaps most effective when placed in cracks and crevices close to infested harborages.

The labels for gel baits include recommendations as to the size and amount of individual bait “placements” and the number of these placements to make in an infested area. However, the location of the placements, not simply the number of placements, is the key to control. The pest control professional must determine the location of cockroach-infested harborages, then apply the bait with one of the special applicator guns or syringes. Once the bait is delivered, the technician may not return for several weeks. If the baits are applied effectively, the client may begin to see the cockroach infestation decrease in a matter of a few days.

The events that occur between the time the bait is applied and the time at which the cockroach infestation begins to decrease are important to a cockroach control program. The key events during this time are that adults and nymphs find the bait and begin eating; and the bait placements begin interacting with the immediate environment. These two interactions may significantly influence control and are important for the pest control operator to understand. Researchers at Virginia Polytechnic Institute, Blacksburg, Va., conducted laboratory tests to learn more about cockroach feeding behavior in response to baits, and how gel baits respond to temperature and humidity in the environment.

A Siege gel bait placement of approximately 0.06 gram.

LABORATORY EVALUATIONS OF BAITS. The German cockroaches used in these studies were collected directly from urban apartments and used in all the evaluations. The gel baits evaluated were Maxforce (The Clorox Company) and Siege (American Cyanamid). Although the ingredients of these two bait products are different, both utilize hydramethylnon (about 2% concentration) as the active ingredient.

The amount of each placement of gel may vary according to the target cockroach and the severity of the infestation. For example, the Maxforce label recommends making placements that are about the size of the syringe cap (¼ gram to ½ gram). The recommended application rate is one or two placements per square yard for small species, such as the German and brownbanded cockroach; and two to four placements per square yard for large species, such as American, smokybrown, and Oriental cockroaches. For heavy infestations of German and brownbanded cockroaches, the two-to-four rate is recommended.

The instructions with the XactaDose applicator for Siege include a plastic card with differently sized circles, representing 0.005 gram to 0.04 gram, that serve as a guide for calibrating the applicator gun for German cockroach control. Further instructions provide the actual number of the different-sized placements to deliver per site of infestation.

The results of the laboratory evaluations showed that to increase effectiveness, bait placements should be located 3 to 4 inches from cockroach-infested harborages. At this distance, cockroaches may be quickly attracted to the bait or they are likely to encounter it when they leave the harborage to forage. Of course, gel baits placed directly in infested harborages stimulate cockroaches to begin feeding almost immediately. Observations showed adults and nymphs move to the bait soon after it is placed close by.

THE FIRST FEW HOURS. Cockroaches are often attracted to new food in their environment, especially if it is moist and pal-atable. Gel baits generally have a high moisture content and a variety of ingredients that attract cockroaches. If cockroaches are in harborages close to where gel baits are placed, they will usually detect the bait within five minutes and move to it.

Bait applications made adjacent to heavily infested harborages may result in a large number of cockroaches gathering around the placement, all trying to gain access to the food. In crowded conditions cockroaches often crawl on top of the placement or contact it with their legs while feeding. Once individuals have fed they move away and usually go back to the harborage. In general, cockroaches take one large meal of these toxic baits at one time (perhaps the result of a feeding stimulant or palatability issue), and may return within the next hour for only small amounts. However, little or no feeding occurs after 24 hours (due to the effect of the insecticide).

In locations with heavy cockroach infestations there may also be a sanitation problem. If there are other more desirable foods available in the immediate environment, baits may be less attractive to cockroaches. Baits can be a key component in a control program but the program must include sanitation.

The attractiveness of gel baits to cockroaches may be linked to several of the ingredients, including the high (as much as 50%) moisture content. But moisture is not responsible for all of the attraction. To evaluate the role of moisture in bait attractiveness, we provided cockroaches with gel baits that were one to several days old, to as much as 10 months old (kept in a household environment). The results were that cockroaches came to and readily fed on all these baits, in spite of the fact that some of them had lost most of their original moisture.

A hungry cockroach will spend several minutes feeding on fresh (one to three hours) gel bait. This extended feeding may be due to hunger, but may also be due to aspects of the bait formulation. For example, baits often contain specific ingredients that stimulate cockroach feeding and act to keep the cockroach eating for an extended time. The result is that a cockroach adult or nymph consumes a large amount of the bait at one time. The amount of food consumed depends on whether it is a male, a female with and without an eggcase, or a nymph.

Male cockroaches forage for food and water regularly. They will usually eat whenever they encounter suitable or attractive food, and this may be several times during the day and night. The amount of fresh gel bait (Maxforce) eaten by a hungry male cockroach is about 3.5 mg While this may not seem like very much, this amount of bait contains about 15 times the dose of hydramethylnon necessary to kill the cockroach.

During the time female German cockroaches are carrying an eggcase they eat and drink infreqently and remain relatively inactive in a protected harborage. Soon after they deposit their eggcase and for the next five to six days before the formation of the next one, females are typically very active and consume large amounts of food and water. The amount of bait eaten by females during this period is 2 to 3 mg, and most of the feeding occurs during the first day after depositing the eggcase.

Small and large nymphs are also attracted to gel baits. They will eat bait that is fresh and bait that is several days old. First-stage nymphs will readily eat fresh bait and they will eat bait that is several days old. The rumor that they don’t eat bait that is more than 24 hours old because of the thin film that forms on the surface is simply not true. The amount of bait eaten by large nymphs ranged from 0.3 mg to 1.3 mg.

A Maxforce Roach Killer Gel Bait placement of approximately 0.5 gram, or the size of the syringe cap

A FEW DAYS LATER. After feeding on gel bait the cockroach may return to the harborage or eat a small amount of other material in the immediate area. For the first 24 hours after feeding the activity of the cockroach is relatively normal and there is little sign of toxicity. Out of perhaps 10 cockroaches that fed on toxic bait, only two may be knocked down by 24 hours.

During the 24 hours after application, gel baits may loose 30% to 40% of their moisture content. The amount of loss depends on the temperature and humidity in the immediate area; for example, the warmer and drier the room, the more moisture that is lost from the bait. This loss of moisture has a significant influence on the concentration of insecticide in the bait. Consider this: a 50% weight loss in a bait that originally contained 2% insecticide results in a bait that now contains 4% insecticide (double the original concentration).

Under the conditions (65% relative humidity and 67°F) of the laboratory test, Maxforce gel bait had a weight loss of 43% in the first 24 hours. This increased the concentration of hydramethylnon in the bait from 2.0% to 3.8%. Although the concentration of the insecticide increased, the bait remained attractive to adults and nymphs. Cockroaches readily feed on gel bait that is 24 hours old and may eat nearly as much as they would from fresh bait. The moisture content alone is apparently not responsible for the attractiveness of gel baits to adults and nymphs.

Male cockroaches ate 3.0 mg of Maxforce gel bait that was 24 hours old, compared to the 3.5 mg eaten when the bait was fresh. Because of the increased concentration of insecticide in one-day-old bait, the cockroaches received about 20 times the lethal dose of insecticide with this meal.

Within 48 hours after feeding on a toxic bait, the cockroach begins to show signs of hydramethylnon poisoning. Movement is very slow, there is a general decrease in walking and foraging, and the legs and wings may begin twitching. Although they are not technically dead, these cockroaches may be so affected by the insecticide that they do not leave the harborage (and are not seen by the client).

The bait may continue to lose moisture during the second day, and this results in a further increase in the concentration of insecticide. The Maxforce gel bait that was evaluated lost an additional 7% of its weight after 48 hours to make a total of 50% lost in two days. The concentration of hydramethylnon in the bait increased to 4%.

Cockroaches were attracted to and ate two-day-old bait, but the amount consumed decreased to 1.2 mg per cockroach (down from the 3.5 mg of fresh and 3.0 mg of one-day-old bait consumed). However, even with eating this small amount of bait the cockroaches received about 10 times the lethal dose of hydramethylnon.

Cockroaches begin to die within 72 hours after consuming the toxic gel baits that include hydramethylnon. About one half of them will be knocked down by this time, some will be dead and many others of those remaining will be inactive and will die within the next 48 hours.

Moisture loss from gel baits essentially stops after two days and there is usually no further change in the concentration of the insecticide in the bait. The bait may remain attractive to cockroaches during the next several days.

Gel baits apparently remain attractive to cockroaches for extended periods. In our tests, male German cockroaches were attracted to bait placements that were 10 days old.

The Siege gel baits remained attractive to cockroaches for as long as 10 months. In our tests cockroaches approached the bait within about 10 minutes after being exposed to it and consumed normal amounts. There was little or no change in the weight or moisture loss during the 10 months.

PRACTICAL APPLICATIONS. The researchers determined the approximate amount of bait consumed by male, female and nymph cockroaches when the cockroaches encountered fresh (24 hours) bait. This information may be helpful to the pest control operator when selecting the size (amount) of bait placements for the gel baits Siege and Maxforce.

The Maxforce Roach Killer Bait Gel utilizes a syringe to dispense the bait in the form of a bead or spot. The recommended size spot is equal to the size of the cap on the syringe, or a placement of about 250 mg to 500 mg, which is about ¼ gram to ½ gram (see photo on page 51). During the first 24 hours, a single ¼ gram placement of Maxforce has the potential to feed and kill 20 males, 20 females without eggcases, and about 80 large nymphs.

The XactaDose Precision Baiting System is a “gun” device that provides for pre-measured applications of the Siege Gel Bait. The owner’s manual and calibration card that accompany the gun provide instructions and a selection of five dose sizes for German cockroach control. The 0.04 gram dose is the largest recommended (for “small roaches”), and the application guide suggests 125 to 188 placements of this size per home. The 0.005 gram dose is the smallest recommended, and 1,000 to 5,000 doses per home are recommended. However, one placement of this dose may be completely consumed by one adult or nymph. During the first 24 hours, six placements of the 0.04 gram dose have the potential to feed and kill 20 males, 20 females without eggcases, and 80 large nymphs.

By understanding the changes that occur in gel baits during the first 72 hours following delivery, such as the loss of moisture and the increase in concentration, and the feeding behavior of cockroaches attracted to bait, the professional pest control operator may be more efficient and effective in using these materials. Some of the myths associated with gel baits, such as small nymphs not feeding after 24 hours, or that baits become “old,” dry and unattractive because they have lost moisture, appear not to be true. However, the use of baits in a cockroach control program should also be accompanied by other control strategies, including sanitation.

Iskandar Ajjan and Dayu Zhang are visiting scientists at the Urban Pest Control Research Center at Virginia Polytechnic Institute, Blacksburg, Va. Ajjan is from Tishreen University, Syria; and Zhang is from Zhejiang Agricultural University, China. William Robinson is a professor in the Department of Entomology at VPI.