Editor’s note: In an attempt to thoroughly cover this complex topic, PCT magazine interviewed a wide range of university researchers and manufacturers’ representatives about insecticide transfer during the past few months. A diverse range of opinions were expressed in the article, which we hope will generate additional discussion among pest management professionals as they interact with their colleagues, as well as university researchers, manufacturers’ representatives, extension entomologists and industry distributors about the role of transfer in modern-day IPM programs.
If you read the marketing literature, scientific studies and attend any number of industry meetings, you’ll get an earful about insecticide transfer. Does it actually occur in the field? How important is it for achieving control? Is it marketing hype? Though PCT magazine certainly doesn’t have all the answers on this highly debated topic, we can provide some practical insights.
TRANSFER PRIMER. Transfer occurs when an insect receives a dose of active ingredient — either by eating bait or contacting a treated surface — and passes this material on to others.
It takes place among social and semi-social insects — ants, termites, cockroaches and yellow jackets — which are in constant physical contact with one another. They may exchange food and fluids (trophallaxis) by mouth or rear end; eat others’ feces (coprophagy), secretions such as vomit (emetophagy), or dead nest mates (necrophagy); carry their dead to a “bone pile” (necrophoresis); rub up against others; perform social grooming; and on some occasions, cannibalize one another.
How transfer occurs, the degree to which it happens, and the importance it plays in achieving control differ from pest to pest and from product to product. Similarly, researchers also differ in their beliefs about transfer and the extent to which it occurs.
TERMITES. The transfer of insecticide from termite to termite can occur via termite baits or liquid non-repellents.
The transfer of bait among termites is accepted as fact because insecticide transfer is required for baits to work. Termites eat the bait, share it through trophallaxis and grooming, and all that ingest a dose of the active ingredient die when they are unable to molt.
In general, the slower the mode of action, the greater the potential for transfer throughout the termite population. “In order to be effective, the active ingredient in a termite bait must be non-repellent and slow acting. The toxicant within the bait must be slow acting so that ‘intoxicated’ termites die at random locations within their foraging territory,” according to the Eighth Edition of the Mallis Handbook of Pest Control. “If the bait kills too quickly, sick or dead termites may accumulate in the vicinity of bait stations, increasing the likelihood of avoidance by other termites. Delayed action also enhances transmission of the toxicant to other colony members, including those that never fed on the bait.”
“With all of today’s emphasis on transfer of liquid termiticides, we sometimes forget that baits were the first materials to capitalize on this characteristic,” said University of Kentucky Entomologist Dr. Mike Potter.
Since the advent of liquid non-repellent termiticides, subterranean termites and horizontal transfer have been a hot topic. These materials adhere to the termites’ cuticle when the pests pass through treated soil. Termites also get an oral dose by moving treated soil with their mouths, grooming one another and through trophallaxis.
Termites exposed to high concentrations often die too quickly to transfer the toxicant to others. At low doses, termites may not pick up enough active ingredient to “pay it forward.” A just-right dose kills the donor slowly so it can unknowingly pass termiticide on to nest mates. But the extent to which this happens and its importance in achieving control are up for debate.
In the case of liquids, some university researchers believe transfer plays a significant role in eliminating termite colonies. Others say it’s an incremental benefit. And still others feel it doesn’t occur at all: Rather, termites pass through the treated zone and die, causing the colony’s other foragers to ignore dead tunnels for different ones.
Researchers also disagree on how far transfer can occur from the treated zone. Some say it’s a relatively short distance, while others cite field studies that found colonies were eliminated significant distances from treatment. One study using DNA probes found colonies in structures and yards vanished, including those unrelated to the structural infestation, following treatment.
All of the researchers PCT interviewed agree transfer can be demonstrated in the laboratory, but because of contrived conditions, some researchers say these findings prove little in the field. The impact of soil pH, temperature, humidity and other field conditions require further study, they state, as does termite biology, social structure, communication and collective intelligence.
Most researchers do, however, agree on one topic: These slow-acting, highly active, non-repellent perimeter-oriented products work. They offer “a degree of forgiveness, a bigger footprint,” said University of Kentucky’s Potter. They give professionals “confidence to deliver what they’ve promised their customers.”
“As long as termites attacking a structure are controlled, and you prevent new termites from getting into the structure, then you’ve achieved the main objective of the treatment,” said North Carolina State University Entomologist Dr. Ed Vargo.
Bayer Environmental Science Product Development Manager Dr. Byron Reid agreed. “Transfer is only a means to an end,” he explained. Eliminating future threats to a structure’s integrity and a pest management professional’s customer guarantee is the ultimate goal.
BASF Market Development Specialist Robert Hickman believes the degree of transfer that occurs in the field varies, depending on a number of factors. He refers to “donor” termites — those that have already come in contact with termiticide-treated soil, and “recipients”— those that haven’t, noting both are key parties in the process of transfer. “There are a lot of different ways transfer of insecticide can occur,” Hickman said, citing such examples as trophallaxis, necrophagy and social grooming.
He likens termiticide transfer to a person contaminated with a communicable disease. “The more you have a chance of interaction between donors and recipients, the greater the chance of transfer, and the degree of transfer is variable.” He notes that whether transfer does or doesn’t occur depends on such things as the frequency and duration of exposure of donors to a treatment, the frequency and duration of interaction between donors and recipients, and the type of transfer mechanism.
Keeping the goal of control in sight, experts remind professionals to follow the label. University of Georgia-Athens Entomologist Dr. Brian Forschler said he doesn’t believe perimeter non-repellents transfer among termites, but “I certainly believe those materials are effective if used according to label.”
University of California-Riverside Entomologist Dr. Mike Rust suggested treating cracks in the slab, brick, or mortar where termites can gain access. “When these entry points aren’t treated during perimeter treatments, field research shows you’re not necessarily going to get good control.” Pest management professionals should take a thorough approach and use a little more material to prevent callbacks, he said.
FMC Professional Solutions Product Development Manager Dr. Dina Richman agreed. She emphasized using a volume of finished liquid appropriate for the specific situation. For example, she says, soil type must be considered when treating for subterranean termites. Also, for a general perimeter treatment to penetrate thick organic layers, like mulch, total volume used should be on the high end of the allowable label range. “If you don’t get the chemical to where some of the insects are, there won’t be any transfer,” Richman said.
In short, experts say, pest professionals should focus on fundamentals: Perform good inspections, apply continuous treatments around structures and don’t short rates, said Bayer’s Reid. “Don’t rely on technology as a crutch to help you overcome weak practices.”
Knowledge of termite biology, foraging behavior and colony structure helps professionals apply baits and liquids correctly and communicate with customers, said Dow AgroSciences Product Technology Specialist Dr. Marc Fisher. “The more educated the customer is about termites and their biology, the better their choice will be in the products they choose.”
Proper interpretation of horizontal transfer is crucial, stressed Auburn University Entomologist Dr. Xing Ping Hu. “It is a simple scientific fact that slow-acting non-repellent termiticide can be transferred between termites, a process that will also dilute the toxic doses. It can be made very complicated or even misleading if we try to make more of it than it can offer. After all, a termite can be killed only after it acquires fatal doses.”
Others feel more strongly. “The marketers need to be cooled off,” said Texas A&M Entomologist Dr. Roger Gold.
The EPA also is paying close attention to transfer talk. “They’re really policing (the industry) on our label claims,” said FMC’s Richman.
COCKROACHES. Transfer of bait among German cockroaches also is a hot topic. Due to the pests’ close living quarters and penchant for eating feces, vomit and dead brethren, transfer of insecticide is generally accepted.
But here’s the rub: What happens in the lab may not mimic what happens in the field. “We know in the laboratory we can get cockroaches to show secondary and tertiary kill,” said North Carolina State University Entomologist Dr. Coby Schal. “We know in the laboratory we can cause this cascade of death, but there’s not a single study that has established this thing happens in the field under real-life (conditions).”
That’s not to say transfer in the field doesn’t happen — it’s just difficult to prove given the multitude of variables, such as the many available sources of food.
Unlike lab roaches, those in the field are not starving, said Whitmire Micro-Gen Senior Research Scientist Dave Naffziger. “They don’t have to eat each other. They’ll eat nymphs and things like that, but they don’t have to eat feces and they don’t have to eat dead roaches,” unless the population is “gigantic,” he said.
Naffziger said he believes transfer plays a minor role in controlling roaches. Primary kill, he said, may be more than adequate to eliminate the infestation.
“The most impactful way (a bait) kills cockroaches is they eat it and it kills them,” agreed Bayer Environmental Science Market Manager Gordon Morrison. When the application is done correctly, with bait placed in close proximity to cockroaches, says Morrison, there shouldn’t be “a lot of roaches left to be controlled via secondary kill.” Morrison adds that secondary kill is more likely to occur further from bait placement in hard-to-reach areas.
But transfer can help control difficult-to-reach cockroaches that live deep in wall voids or under layers of wall or flooring materials. Products that kill additional roaches with just one feeding give pest management professionals the confidence to control infestations, said DuPont Professional Products Global Development Manager Dr. Mark Coffelt.
And, good sanitation improves the odds. “The better the sanitation, the more likely roaches will feed directly on the bait, the feces and vomit of other roaches,” which will improve the opportunity for secondary kill, said Morrison.
Environmental conditions, population pressure and the population’s history with baits also play important roles, said Coffelt. “All of these factor into how successful (transfer) may occur in the field.”
Transfer does, however, raise concerns. For example, as the pests move from below the sink to counter top, so does the insecticide. “From the perspective of environmental toxicology, translocation is never good,” said Schal, even if the amount of insecticide moved is “infinitesimally small.”
Naffziger agreed. “If you can transfer it in a crack and crevice, you can transfer it out in the open, and do we want roaches transferring things out where kids are exposed?”
Transfer also could open the door to bait resistance. Once ingested, insecticide is metabolized or degraded by the roach, so others eating that roach’s feces could ingest a lower concentration of insecticide. Some may not die. “That is precisely the beginning of insecticide resistance,” said Schal. Resistance can be minimized by performing thorough inspections to determine proper bait placement, improving sanitation practices to reduce competing food sources, and applying bait according to the label rate.
ANTS. Insecticide transfer among Argentine, fire and carpenter ants, whether from perimeter spray or bait, is generally more accepted. First, ants carry their dead and deposit them in cemeteries. “In that process they’re coming in contact with ants that could have been exposed to a treated barrier or bait,” said UC-Riverside’s Rust. “Right there, there’s an opportunity for an insecticide to be transferred.”
Second, ant colonies have a true social stomach. Foraging adult female ants, which are unable to swallow solid food and represent one to 10 percent of the colony, carry food back to the nest. This food is fed to larvae, which chew the solids and defecate. These remains are fed to other adults. “There’s a much better flow of food through the colony than (with) termites, who have a diffuse network of numerous feeding sites that everybody can eat,” explained University of Georgia-Athens’ Forschler.
To help foster transfer among Argentine ants, Rust suggested lightly spraying trailing ants and spot treating satellite nests. “You’re going to increase the likelihood that those ants are going to return to the colony or interact with other ants before they die.”
University of Georgia-Griffin Entomologist Dr. Dan Suiter agreed. “We used to tell the technicians, ‘Don’t spray the trails or you won’t get material back to the queen.’ Now, I tell technicians spray every trail you can find.”
For carpenter ants, apply perimeter sprays under the lower edge of siding around structures, suggested Spokane Falls Community College Entomologist Dr. Laurel Hansen. “It lasts longer there because of protection from direct sunlight and rainfall. These factors contribute to photo decomposition or removal. The materials therefore remain active for a longer period of time.” And don’t overlook window and door frames, she said.
Baits take advantage of ants’ unique behavior and changing preference for proteins and carbohydrates, explained DuPont’s Coffelt.
Proper placement remains vital. “You want to make sure you’re placing the bait in a location where the ants are going to pick it up,” since ants will recruit nest mates, explained Syngenta Technical Representative Dr. Deanna Branscome.
Educating customers about transfer ultimately helps professionals do their job. “Once you explain to them how the transfer works and the benefits of it, then they’re willing to wait a couple days” for the product to take effect, Branscome said.
YELLOW JACKETS & OTHER PESTS. Researchers at University of California-Riverside are working to exploit the use of baits and horizontal transfer to control yellow jackets. Like ants, there may be a way to get yellow jackets to take a toxicant back to the colony and spread it throughout. Although studies are continuing, “We’ve had some success in being able to show at least the concept will work,” said Rust. “Now it’s just a matter of finding the right insecticides and materials.”
Bed bug control also may benefit from horizontal transfer. Although not social insects, bed bugs hang out in brood centers, rub up against each other and move over old cast skins. “Although no research to date that I’m aware of demonstrates this, there’s a potential for something like this to occur depending on the product,” said BASF’s Hickman.
MOVING FORWARD. Eventually, scientific field studies may settle the transfer debate. Until then, experts say stay true to Integrated Pest Management and apply products in accordance with the label.
“To my knowledge, pest control operators really don’t modify their operational procedures to maximize secondary kill for any insect treatment,” said North Carolina State University’s Schal. Many experts suggest professionals view horizontal transfer as an added bonus — a control enhancement.
Just don’t overplay its promise, which environmentalists could misinterpret as drift or food chain involvement, cautioned Texas A&M’s Gold.
The impact of transference in the field remains elusive, said University of Kentucky’s Potter. “The degree to which it exists may be irrelevant to the operator since there’s no question the products are performing terrifically.”
The author is a frequent contributor to PCT magazine.
Researchers Sound off on Transfer
About Termites…
“I think there is a little bit of transfer from grooming, cannibalism and so forth, but whether or not it is important in terms of pest control in the field, I question it.” — Dr. Roger Gold, Texas A&M
“I think it’s clear there’s evidence there is transfer, whether or not it’s enough to eliminate whole colonies is up in the air. My feeling is in many cases it is.” — Dr. Ed Vargo, North Carolina State University
“Under certain conditions, you’re going to get some movement of the termiticide. It can happen, but when you get down to modeling all the things that have to fall in place for that to happen in the real world, it’s going to be a rare occurrence.” — Dr. Brian Forschler, University of Georgia-Athens
“Toxic transfer can cause a catastrophic chain reaction and result in depression or even elimination of the termite population. It is indeed a chain reaction, not a one-time event.” — Dr. Ping Xing Hu, Auburn University
“Several researchers have demonstrated transfer from insect to insect, but just how much (transfer) is contributing to the overall impact that you get, we really don’t know.” — Dr. Mike Potter, University of Kentucky
“If transference were really true on a huge area-wide basis, you should be able to spot treat…because the material is going to have a wide-area impact.” — Dr. Mike Rust, University of California-Riverside
About Ants…
“Folks talk hand-in-hand about transfer and non-repellency. I think, at least in ants, it’s probably more important that the product be slow acting. The mere fact of the live ant picking up the dead ant is enough to transfer the material to kill the live ants. You get significant mortality doing that.” — Dr. Dan Suiter, University of Georgia-Griffin
“We’ve been able to show, at least with fipronil, that (transference) does occur and probably does have a significant impact on control. Transference in ants is potentially more important than maybe it is with termites.” — Dr. Mike Rust, University of California-Riverside
About Cockroaches…
“No one has ever gone to an apartment or restaurant and documented that secondary kill has any function, any type of role, under real-life conditions.” — Dr. Coby Schal, North Carolina State University
Entomology-Speak
Everyone knows the meaning “contact” but in addition to traditional contact/consumption methods, there are a wide range of other ways insects can transfer insecticides.
Social insects share some interesting habits, but even stranger words describe these behaviors. Here’s a cheat sheet to help you prepare for your next industry get-together:
- Coprophagy (ko-pro-fay-je) — The eating of feces.
- Emetophagy (e-met-o-fay-je) — The eating of secretions, such as vomit.
- Necrophagy (nek-row-fay-je) — The eating of the dead.
- Necrophoresis (nek-row-for-e-sis) — The removal of the dead by carrying them to places away from the living.
- Trophallaxis (trofe-a-laks-is) — The transfer of food and fluids among members of a community by mouth-to-mouth or anus-to-mouth.
Marketing Speak
Here’s a look at how manufacturers explain the transfer phenomenon:
Editor’s note: Manufacturers, if we’ve missed your company and you’d like to comment on the transfer of insecticides, contact PCT Editor Jodi Dorsch at jdorsch@giemedia.com and we’ll be sure to include your comments in an upcoming issue of PCT.
Domino Effect® and ContactX™ — Bayer Environmental Science’s Maxforce® cockroach and ant baits. To make matters worse for roaches, Maxforce’s exclusive Domino Effect® delayed killing action enables one contaminated roach to kill many others where they live and breed. A foraging roach is guaranteed to receive a lethal dose from a single feeding, and lives long enough to return to the harborage where it unknowingly shares the active ingredient with other roaches. Additionally, Bayer’s ContactX technology with Maxforce FC Magnum does not require cockroaches to consume the bait to die; they simply need to touch it. The bait matrix was specially designed to be attractive and consumed by both normal and finicky cockroaches, though an actual meal is no longer required to achieve control. ContactX technology also provides secondary kill via contact alone.
Transfer Effect™ — BASF Termidor® termi-ticide. Thanks to Termidor’s unique “Transfer Effect,™” termites don’t even need to ingest or contact the termiticide in order to die from it. All they need to do is make contact with another termite that has already contacted or ingested Termidor. This is because termites that come into contact with Termidor collect it on their bodies. They then become carriers that spread the termiticide to other termites. And the “Transfer Effect” doesn’t quite end there. Secondarily affected termites can also pass Termidor on to others and indeed may wipe out the entire colony. Termites are social insects that live together in large colonies and feed and groom each other. The “Transfer Effect” makes the most of this.
Recruit™ termite bait — Dow AgroSciences does not have a tag line to describe horizontal transfer. Rather the name of its Recruit™ termite bait implies how the bait is fed upon, how termites enlist other members of their colony to feed on it, and how fed termites transfer the bait to nest mates through trophallaxis, said Product Technology Specialist Marc Fisher.
Advion®. True Exponential Control.™ — Researchers at the Department of Entomology at Purdue University have proven DuPont™ Advion® cockroach gel to be the first U.S. product to provide tertiary kill. Exponential control is a three-step chain reaction in which one cockroach’s exposure to a toxicant is multiplied into killing a secondary and then tertiary group. According to DuPont Professional Products, this is crucial for thorough infestation elimination since pest management professionals using traditional methods of treatment cannot reach first instar nymphs and gravid females that typically remain in the harborage area.
Pass it On™ — FMC Transport® wettable powder termiticide. The speed of control of Transport comes from termites sharing, or passing on, the active ingredient among other termites in the colony, while its optimized formulation offers long-term protection of the structure. Transport’s tag line refers to the physical movement of active ingredient from termites mechanically moving treated soil, said Product Development Manager Dr. Dina Richman.
Optigard as a foam insecticide — Optigard applied as a foam fills wall voids where these insects make their homes. This non-repellent formula uses the active ingredient thiamethoxam, which pests then transfer to others. “The concept of transfer is so familiar to our customers that Syngenta didn’t feel like we needed to specifically have a tag line,” said Technical Representative Deanna Branscome. “When we do describe the efficacy of our non-repellent chemistry we do let our customers know the product is readily transferred throughout a nest or a colony.”