Part 2 in our Active Insight series looking at how to get the best results with cockroach baiting.

Urban environments contain a wide variety of complex man-made structures that provide a plethora of harbourages and resources to allow cockroaches to establish an infestation.

Studying how cockroaches adapt and behave in these environments allows us to better understand cockroach behavioural characteristics and improve our bait placement techniques.

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 both the cockroaches feeding preferences, as discussed in Part 1, and their foraging and social interactions and habits, we can maximise both the attractiveness of our baits and their placement.

We are all aware that cockroaches are largely nocturnal. Typically, they depart their daytime harbourage sites to forage for food and water and to search for sexual partners in the early evening. In more natural environments, cockroach activity tends to peak around 2-4 hours after sunset. In our man-made environments, these ‘normal’ circadian rhythms will change to suit the situation. So, for example, if we have a restaurant that is open until late in the evening, with lights blazing, it will be around two hours after the lights go out that activity will peak.

Most behavioural studies have been performed using the German cockroach (Blattella germanica Linnaeus) as the study model. Unless stated otherwise in this article, when referring to cockroaches, this will be the specific test species, though as a broad generalisation, we can expect similar behavioural traits to be present in Periplaneta species and other commensal genera.

Given the simple nerve ganglion structure that represents the cockroach brain, it is perhaps surprising to realise the complexity of the cockroaches’ activities and the ‘knowledge’ they develop with respect to their environment.

Studies by researchers such as Durier and Rivault1 have shown that cockroaches are fully aware of the spatial distribution of food and water in their localised environment. Contrary to the advice often extolled, these studies show that cockroaches do not forage randomly, in fact they are highly efficient in their travels between food and water sources and their harbourages.

Whilst cockroaches live in an ‘aggregate’ (a loose social structure), they forage in their environment individually, using knowledge that they each ‘learn’ for themselves. They do this through a process known as path integration, employing their prior exposure to various odours and visual cues from earlier foraging excursions, to develop their own navigation system.

Path integration can be considered as a system whereby a cockroach continuously updates its knowledge base, with the direction and distance to a new feature in their harbourage arena,2 be that food, water or shelter. The recollection of a favourable food source at a specific location is usually linked to olfactory and visual cues, such as food odours and objects around the food source. At Ensystex we have exploited this when developing our various cockroach baits. In addition to including the required high nutrient ingredients and values as discussed in Part 1, we include strong olfactory stimulants that we know are favoured by the cockroaches.

In a new environment, when stimuli from food odours and learned visual cues are absent, German cockroaches explore in a random pattern that covers all accessible surfaces in the surrounding environment. Once they find a preferred food source, they will integrate its position into their navigational database, linking it to the learned visual and olfactory cues, and then relating its position in terms of both direction and distance to their harbourage.

On future foraging expeditions, they will then travel directly, more or less taking the shortest route, to the memorised location. This greatly enhances their foraging efficiency.

Cockroaches tend to place a greater emphasis on olfactory cues when seeking out food sources, and visual cues when returning to their harbourages.

The foraging activity and behaviour of cockroaches is determined by the age of the cockroach. Cockroaches at different instars forage differently. For example, early instar nymphs forage in a similar manner throughout those stadia, whilst fifth instar nymphs show the path integration patterns and evolve more efficient foraging behaviour.3

It appears that when they first hatch, first and second instar nymphs rarely leave their harbourages. Only as they develop into later instars, do they venture further afield. By the fifth instar they are highly active in their foraging and path integration.

This is an important aspect to note from the perspective of cockroach baiting. In naturally occurring cockroach populations, around 60% of the population consists of nymphs and this is largely unaffected by population size. If a baiting program is to be successful, it is vital that we take out all the nymphal stages, particularly the first and second instars that might not be targeted by a standard gel baiting program.

Given that cockroaches establish these intricate networks to connect their food and water sources with their harbourage through this path integration process, we need to review how our baits will be perceived.

Sticky monitoring traps should be a key part of a cockroach baiting program

Durier and Rivault also showed that cockroaches would feed on a gel bait in preference to a prior food source, when it was placed in a new location, at about the same distance from their harbourage as their current feeding source. In this study it was 600mm away from the harbourage, but near a visual landmark known to the cockroaches.

More notably, when the gel bait was used to replace the original food source at the same location, and also when the gel bait was placed further away than their original food source, the cockroaches largely ignored the gel bait. Thus, location of baits is important regardless of the palatability of the bait. Clearly when we replace the existing food with a gel bait, the cockroaches note a discrepancy and bait avoidance is the likely result. Presumably they identified this localised change as a potential threat, and changed their behaviour to explore and forage for a more familiar food source that matched their stored information. The logical conclusion from this is that baits should be placed as close as possible to cockroach harbourages.

Indeed, various studies4 have demonstrated that gel baits applied in many small drops at multiple locations provide greater control efficacy than simply placing a few large drops or smears of bait. The latter practice being something I sadly see too often, as it is of course a quicker/easier option.

One clear reason for needing to place several baits is to avoid aggression behaviour among cockroaches. Dominant cockroaches will aggressively protect a favoured food source and attack early life-stage cockroaches or less vigorous individuals to defend ‘their’ food source. Whilst these dominant individuals will later die from the toxin; due to the previously detailed path integration process, those cockroaches that were chased away, are unlikely to return, as they will have added that experience to their knowledge base and stay clear of the area in the future.

Baiting programs can therefore be best enhanced by using monitoring traps to identify the location of cockroach harbourages and then placing baits near these clearly identified locations.

An alternative approach is to supplement gel baiting programs with a dry flowable bait, such as Magnathor, which can penetrate deep into harbourages; or to use a liquid bait formulation, such as Attrathor. Both these options can be applied to larger areas for improved control.

Part 3 of this series on cockroach baiting will we explore the various aspects involved in the horizontal transfer of cockroach baits.

Steve Broadbent, Regional Director, Ensystex

1 Durier, V. and Rivault, C. (2001) Effects of spatial knowledge and feeding experience on foraging choices in German cockroaches. Animal Behaviour 62, 681-688.
Durier, V. and Rivault, C. (2002) Importance of spatial and olfactory learning on bait consumption in the German cockroach. In: Jones, S.C., Zhai, J. and Robinson, W.H. (eds) Proceedings of the Fourth International Conference on Urban Pests, Charleston, South Carolina, 7-1 0 July 2002. Pocahontas Press, Blacksburg, Virginia, pp. 59-64.

2 Collett, T.S. and Graham, P. (2004) Animal navigation: path integration, visual landmarks and cognitive map. Current Biology 14, R475-R477.

3 Demark, J.J., Kuczek, T. and Bennett, G.W. (1993) Laboratory analysis of the foraging efficiency of nymphal German cockroaches (Dictyoptera: Blattellidae) between resource sites in an experimental arena. Annals of the Entomological Society of America 86, 372-378.

4 Durier, V. and Rivault, C. (2003b) Improvement of German cockroach (Dictyoptera: Blattellidae) population by fragmented distribution of gel baits. Journal of Economic Entomology 96, 1254-1258.