Part 3 in our Active Insight series looking at how to get the best results with cockroach baiting.
The transfer of cockroach baits has been well documented in the last twenty years, and is generally considered to play an important role in cockroach management programs.
Transfer is considered to rely on the cockroaches contaminating their environment with toxicant laden cadavers, faeces and vomitus. These cockroach by-products are then fed upon by other cockroaches, which are subsequently intoxicated by the selected toxicant.
Although secondary kill by cockroach baits has been reported in various studies, its relative importance in overall cockroach mortality is often misunderstood.
The transfer of cockroach bait toxicants has arisen over the past twenty years as a consequence of improved bait technologies; the development of non-repellent active constituents such as hydramethylnon, abamectin, fipronil and, more recently, indoxacarb; combined with improved knowledge of cockroach biology and foraging ecology.1
For the modern professional pest manager it is generally accepted as being one of the key reasons for employing baiting technologies in the control of cockroach aggregates. A range of tradenames have been placed in the market to convey the importance of transfer, such as ‘Domino effect’, ‘Viral transfer’ and ‘Transfer effect’. As a consequence, a degree of mystique and belief systems have evolved within our industry, that are perhaps not supported by the scientific data, when we face cockroach management issues in the field.
Toxicant transfer is most dependent upon those cockroaches that have fed upon an intoxicated bait contaminating their immediate environment with toxicant laden faeces and vomitus, which is then consumed by other cockroaches, leading to their demise. This transfer or secondary distribution of the toxicant is considered to be most important in the control of the sedentary life-stages in a cockroach population, specifically gravid females and early instar nymphs, especially first instar nymphs that do not venture far out of their harbourages.2
Two main mechanisms for the transfer of actives are evident from a practical perspective.
Coprophagy refers to the consumption of cockroach faeces and proctodaeal feeding (from the anal region). In the natural environment, coprophagy is essential for survival and is the process by which cockroaches transfer hindgut symbionts and other nutrients within the cockroach aggregate. Coprophagy is evident in all life-stages, though it is most important for first instar nymphs. One study found that 90% of newly hatched nymphs could survive for ten days, with 70% surviving for fourteen days, by consuming adult faeces only.
Of more relevance to cockroach survival, nymphs that were denied access to cockroach faeces died sixty times faster than those nymphs that were able to partake in coprophagy. In this case female faeces was found to be more nutritious than male faeces.3
Cockroach faeces is known to contain pheromones that can serve to aggregate cockroaches in their harbourages. Consumption of faeces by early instar nymphs and gravid females allows them to secure nutrients without the need to venture far from their harbourage, which is an adaptive behaviour to aid survival. As cockroaches mature their dependence on coprophagy as a food source diminishes.
To be successful in the transfer of the toxicant to other cockroaches, it is essential that the active constituent remains stable as it is passed through the cockroach digestive system. The other important factor for success in the distribution of the active in this manner is that the active must not kill the adult cockroaches too quickly. The cockroaches need adequate time to return from their foraging trips to their harbourages. If the active kills them before they return, this transfer is not going to happen.
The importance of coprophagy in ‘real world’ scenarios is questioned. Male faeces is less attractive than female or nymphal faeces from a consumption perspective. 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 instar nymphs. With later life-stages, whilst coprophagy is documented, there is little to suggest that the cockroaches prefer faeces to normal food or that concentrations of toxicant in adult cockroaches are going to provide control in a timely fashion.4
However, it has been observed that indoxacarb-poisoned cockroaches produced a distinct liquid excretion at the end of the cockroach abdomen (not faeces) within 24 hours of bait ingestion6. Exposure to the excretion caused significant mortality to German cockroaches.
This is mostly reported where early nymphal stage cockroaches feed on the vomitus of other cockroaches, particularly the vomitus of females. In contrast to coprophagy, emetophagy is not essential for nymphal survival, but cockroach nymphs seem to really like to feed on female vomitus in particular. It might be considered like ice cream for cockroaches – it is not required for good nutrition, but they like to eat it anyway. The consumption of vomitus is greatest when other foods are not available.
Most published studies have been performed with fipronil where it has been shown that cockroaches vomit about four hours after ingestion of the fipronil intoxicated bait.5 Toxicant laced vomitus is most effective when it is fresh, and the activity declines rapidly over time.
Additionally it is important to consider the availability of vomit and faeces under field conditions. With faster acting bait actives, such as fipronil and abamectin, the poisoned cockroaches will often enter into convulsions and vomit before returning to the harbourage, preventing cockroach nymphs from accessing fresh vomitus. In theory slower acting insecticides such as indoxacarb will allow greater numbers of cockroaches to return to the harbourage and therefore allow increased access to vomitus and improved secondary mortality.
However, like coprophagy, the practical effects seem to be mostly towards killing the early instar nymphs.
This refers to the consumption of dead cockroaches and their body parts by other cockroaches. 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. More recently though, it has 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 eating dead cockroaches or cockroach body parts and, from a practical perspective, in the commercial control of cockroaches, its impact should be considered negligible or non-existent. Indeed it has been shown that 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 an insecticide or not. And when cockroaches were provided a choice between an intoxicated cadaver and food, mortality declined significantly.
Although there may be differences in the natural environment, available data indicate that necrophagy is the last resort of starving cockroaches.
So what’s happening in the field?
From the above we can see that, whilst coprophagy and emetophagy play a role in the elimination of the early nymphal life-stages, they are of little significance in the control of adult cockroaches; and that, contrary to the often espoused advice, necrophagy (cannibalism), has no practical control benefit, as there is little or no transfer of toxicant occurring through this behaviour.
This opinion is supported by studies that have shown that the levels of secondary mortality decreased from 100% in the first instar nymphs to as low as 12.1% in adult males. Field strains of cockroaches were also much less susceptible than the laboratory reared strains, with only 9.2-16.6% secondary mortality occurring among 3rd and 4th instar nymphs in field strains.6
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 cockroaches may not eat as much bait as those under laboratory conditions, because of the usually more diverse food sources available, which dilutes the active ingredient concentration found in donor cadavers, faeces, or vomitus. Thus, in the field, the role of secondary kill may be much lower than that tested under laboratory conditions.
Of greater import is the improvement of sanitation, especially the removal of alternative food sources. In any event, cockroach debris (cadavers, body parts, faecal pellets) should be removed, since it is highly allergenic to sensitive individuals.
In conclusion, toxicant transfer is important for the control of early instar nymphs, but it is unlikely to play a significant role in the control of later instar moults or adults. 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 transfer process’ effects on the early instar nymphs.
Part 4, the final instalment in this series, will examine bait aversion behaviour.
Steve Broadbent, Ensystex, Regional Director
1 Rust, M.K., Owens, J.M. and Reierson, D.A. (1995) Understanding and Controlling the German Cockroach. Oxford University Press, New York.
2 Durier, V. and Rivault, C. (2000) Secondary transmission of toxic baits in German cockroach (Dictyoptera: Blattellidae). Journal of Economic Entomology 93, 434-440.
3 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.
4 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.
5 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.
6 Changlu Wang, Yang, X. and 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.