28 May 2019

Creating excellence in cockroach baiting programmes

Pest control | PPC95 May 2019

Steve Broadbent from Ensystex is back in PPC for part two of his cockroach article. Steve’s talk went down a storm at PestEx 2019 with some people calling it the best seminar they’ve ever heard. This time we’re talking about what makes for a precise and targeted baiting programme.

This is the second of two articles on cockroaches by Steve. Check out for ‘Understanding cockroach foraging behaviour’ in the last issue of PPC.


  • To be attractive, bait must contain nutrients that are both limited and unique in the urban area
  • Cockroaches select nutrients to correct deficiencies from feeding on unbalanced food
  • Baits should provide a complete and balanced food component to optimise success
  • Cockroaches eat cockroach faeces to gain nutrients necessary for survival
  • Slower actives provide increased access to vomitus and improve secondary mortality
  • Resistance in a cockroach population may increase if baits fall short of being lethal.

Creating excellent cockroach baiting programmes

Cockroach baiting has become the primary method of control in urban cockroach management programs. Baiting offers a precise, targeted solution that employs significantly lower levels of toxicant, in keeping with the principles of Integrated Urban Pest Management.

It is also perceived as offering a lower risk approach to pest management.

Cockroach nutrition

For a bait to be attractive when placed in urban areas with many competing food sources, it must contain nutrients that are both limited and unique in the environment. The nutritional value of foods has a very significant impact on cockroach development and reproduction.

Given the relatively simplistic nerve structure of cockroaches, with the brain a supra-oesophageal nerve ganglion (cluster of nerves) formed by the fusion of three pairs of ganglia; it is surprising to learn that cockroaches are actually able to self-select the nutrients they require to correct nutrient deficiencies that arise from feeding on unbalanced food sources.

Carbohydrates are mostly consumed during the first week after hatching, ie during the first stadium (moult), with lesser amounts required in each subsequent stadium. In contrast, protein is required during all life stages, though in a lower proportion.

When cockroaches were reared in an environment where they were able to self-select the nutrients required, they grew optimally by selecting the requisite nutrients for each nymphal development stage.

It was also reported [1] that when nutrient deficiencies exist in a food source, German cockroaches (Blattella germanica Linnaeus) prefer to self-select an unbalanced food, provided it will compensate for the nutrients they are lacking from a more balanced food.

Cockroaches proceed to distinguish food of different nutritional value through a process known as specific nutrient learning. They learn which nutrients their body requires most and then seek these out. Associative learning has also been reported in the American cockroach (Periplaneta americana Linnaeus), where the insects were able to associate the smell of a food and associate this to the proteins present and required.

The consequences of this are most important in the development of a cockroach bait since it makes it vital to provide a complete and balanced food component, containing all the nutrients required by the cockroaches, to optimise success.

Typically, three macronutrients are included in the matrices of a quality cockroach bait: carbohydrates, lipids and proteins, though it is essential to include further ingredients to provide a complete dietary food source. Sadly, many baits are based on sugars only which leads to less than optimal results in the field.

Attractants and feeding stimulants are also important with the former required to draw the cockroaches to the bait, and the latter ensuring the cockroaches consume larger amounts of the bait to achieve increased toxicant uptake. Other additives are added to prolong the lifespan and retain moisture content.

Studies have shown some baits are more attractive to the cockroaches, and cockroaches are drawn to them by preference. However, the cockroaches eat significantly less since they feed on these baits for a shorter time period. This is why the targeted addition of feeding stimulants is important.

A german cockroach that underwent ecdysis. It is teneral until its cuticle hardens

Foraging behaviour

Cockroaches have evolved to exploit our modern urban environments where the structures we build provide a plethora of harbourages, warmth, food and moisture. By understanding the cockroaches foraging and social interactions, we can maximise bait placement.

You can read a full article on foraging behaviour by Steve in PPC94 

Toxicant transfer

The transfer of cockroach bait toxicants has arisen through the development of improved bait technologies. At the same time, a degree of mythology has arisen surrounding toxicant transfer, that is often not supported by scientific data.

Toxicant transfer is dependent upon cockroaches that have fed upon a bait, contaminating their environment with toxicant laden faeces and vomitus, which is then consumed by other cockroaches. This transfer or secondary distribution of the toxicant is most important in the control of the more sedentary life stages, specifically gravid females and early stadia nymphs [8].

Coprophagy refers to the consumption of cockroach faeces together with proctodaeal feeding (from the anal region). Coprophagy is essential for cockroach survival since it provides for transfer of hindgut symbionts and other nutrients. Coprophagy is evident in all life stages, though it is most important for first stadia nymphs. Kopanic et al showed that nymphs that were denied access to faeces died 60 times faster than those able to partake in coprophagy; with female faeces being more nutritious than male faeces [9].

Cockroach faeces contain pheromones that serve to aggregate cockroaches in their harbourages, allowing early stadia nymphs and gravid females to secure nutrients without venturing from their harbourage. As cockroaches mature, their dependence on coprophagy as a food source diminishes.

Although faeces from adults fed on baits are clearly toxic to cockroaches, it is most likely that it is only a significant factor in the control of the early stadia nymphs. With later life stages there is little to suggest that they prefer faeces to normal food or that concentrations of toxicant in adult cockroaches are going to provide control in a timely fashion [10].

Emetophagy is mostly reported with early stadia cockroaches feeding on female vomitus. Emetophagy is not essential for survival, but cockroach nymphs seem to really enjoy female vomitus.

Toxicant laced vomitus is most effective when it is fresh, and the activity declines rapidly over time [11]. With faster acting bait actives, poisoned cockroaches will often enter into convulsions and vomit before returning to the harbourage.

In theory, slower acting actives, eg (s)-indoxacarb, will provide increased access to vomitus and improved secondary mortality. Like coprophagy, the practical effects seem to be mostly towards killing the early stadia nymphs.

Necrophagy refers to the consumption of cockroach cadavers or body parts. It was originally considered to be an important aspect of toxicant transfer since it is a behaviour that is frequently observed during the laboratory rearing of cockroaches. It has subsequently been shown that the level of necrophagy is dependent on the quality and quantity of food available, and the density of the cockroach population.

Necrophagy really only occurs when cockroaches are deprived of food.

When other foods are available, it is highly unusual to find cockroaches partaking in necrophagy and, from a practical perspective, its impact should be considered negligible or non-existent. Cockroaches in a natural population are more likely to utilise their own body resources, stored fats, before considering necrophagy. In one series of behavioural assays, cockroaches preferred nearly all alternative foods to dead cockroaches, regardless of whether the cadavers contained a toxicant or not. And when cockroaches were provided a choice between an intoxicated cadaver and food, mortality declined significantly [11].

From the above, we can see that, while coprophagy and emetophagy play a role in the elimination of the early life stages, they are of little significance in the control of adults and, contrary to frequently espoused advice, necrophagy has no practical control benefit. This is supported by studies that have shown that the levels of secondary mortality decreased from 100% in first stadium nymphs to as low as 12.1% in adult males.

Field strains of cockroaches were also much less susceptible than laboratory reared strains, with only 9.2-16.6% secondary mortality occurring among third and fourth stadia field strain nymphs [12].

The opportunity for food choice due to low levels of sanitation reduces the probability for cockroaches to feed on bait-killed cockroaches or their excretions. In addition, field populations may not eat as much bait as those under laboratory conditions due to the more diverse food sources available, which dilutes the active concentration found in donor cadavers, faeces, or vomitus. Thus, in the field, the role of secondary kill may be lower than that tested under laboratory conditions.

In any event, cockroach debris (cadavers, body parts, faecal pellets) should be removed, since it is highly allergenic to sensitive individuals.

Toxicant transfer is important for the control of early stadia nymphs but is unlikely to play a significant role in the control of later stadia nymphs or adults.

It is highly unusual to find cockroaches partaking in necrophagy


There are few documented scientific reports of control failure of cockroach bait products, containing the more recently developed active constituents, due to physiological resistance. When cockroaches feed on a quality bait, the active is consumed at significantly higher doses than those required to kill.

The frequency of a resistance gene within a German cockroach population may increase if some cockroaches survive sub-lethal exposure to baits. This may occur through the ingestion of lower doses of insecticide from cockroach excretions, the use of less palatable baits, or when feeding is interrupted because of cockroach aggression behaviour.

A recent study [13] followed reports of control failure in an apartment in Puerto Rico that had been heavily baited for several years with indoxacarb, fipronil and hydramethylnon gel baits.

This study reported three novel findings:

  • The first evidence for hydramethylnon resistance in any insect
  • Extremely high levels of indoxacarb resistance in a field population
  • Reduced secondary mortality in an insecticide-resistant, field-collected strain of B. germanica.

It was still noted though that bait formulations may remain effective, even when dealing with moderately resistant cockroach populations. The difference in mortality between an insecticide-susceptible strain and the resistant strain isolated in the apartment was much less in bait-feeding tests than from topical application, with most adult resistant cockroaches dead by the end of the baiting trials.

This is related to the fact that bait actives such as fipronil, hydramethylnon and indoxacarb are more toxic by ingestion than by topical application, with indoxacarb activated by insect enzymes.

Also, as mentioned above, ingestion can deliver significantly greater doses of the actives to overcome low to moderate resistance. For example, it was estimated that baiting delivered 600 times the LD50 and nearly 400 times the LD90 of fipronil, which is more than sufficient to overcome the 16-fold resistance reported with the resistant strain.

The key aspect with cockroach baiting is to ensure that plenty of bait is made available, especially when dealing with serious infestations. Selecting a bait that provides high palatability is of primary importance for optimal performance in eliminating cockroach aggregates; with the elimination process aided by the placement of multiple small bait spots in close proximity to cockroach harbourages, and the transfer process’ effects on the early stadia nymphs.


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[9] Kopanic Jr. R.J., Holbrook, G.l., Sevala, V. and Schal, C. (2001) An adaptive benefit of facultative coprophagy in the German cockroach Blattella germanica. Ecological Entomology 26, 154-162.
[10] Appel, A.G., Sims, S.R. and Eva, M.J. (2008) Factors affecting coprophagy and necrophagy by the German cockroach (Dictyoptera: Blattellidae). In: Robinson, W.H. and Bajomi, D. (eds) Proceedings of the 6th International Conference on Urban Pests, Budapest, Hungary, pp. 139-142.
[11] Buczkowski, G. and Schal, C. (2001) Emetophagy: fipronil-induced regurgitation of bait and its dissemination from German cockroach adults to nymphs. Pesticide Biochemistry and Physiology 71, 147-155.
[12] Changlu Wang, Yang, X., El-Nour, M.A. and Bennett, G.W. (2008) Factors affecting secondary kill of the German cockroach (Dictyoptera: Blattellidae) by gel baits. In: Robinson, W.H. and Bajomi, D. (eds) Proceedings of the 6th International Conference on Urban Pests, Budapest, Hungary, pp. 139-142.
[13] Ko, A.E., Bieman, D.M., Schal, C. and Silverman, J. (2016) Insecticide resistance and diminished secondary kill performance of bait formulations against German cockroaches (Dictyoptera: Blattellidae). Pest Manag Sci. 2016 Sep; 72(9): 1778–1784.

Source: PPC95