Pest control | PPC94 March 2019

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

SPEED READ

• German cockroaches are a gregarious species living in mixed-family aggregates
• Cockroaches live in aggregate but forage individually, using the knowledge that they each learn for themselves
• They learn which nutrients their body requires most and then seek these out
• A grouping mechanism leads to the formation of feeding aggregations and the occurrence of collective food selection
• Their foraging activity and behaviour are determined by the age of the cockroach.

German cockroaches (Blattella germanica linnaeus) form a gregarious species. They live in mixed-family aggregates, consisting of roughly equal numbers of males and females, with a typical ratio of 60% nymphs and 40% adults. These aggregates often feed on the same food sources, returning to a communal harbourage after each expedition.

They are largely nocturnal and typically depart their daytime harbourages, to forage for food and water and search for sexual partners, in the early evening. In natural environments, cockroach activity tends to peak two-to-four hours after sunset. In man-made environments their normal circadian rhythm will change to suit the situation, eg in a restaurant that is open until late in the evening, activity will peak around two hours after the lights go out.

PATH INTEGRATION

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, they are highly efficient in their travels between food, water and their harbourages.

While cockroaches live in aggregate, they forage individually, using the knowledge that they each learn for themselves. This is referred to as path integration, with the cockroaches employing their knowledge of odours and visual cues from previous foraging excursions to develop their own unique navigation system.

Path integration can be considered as a system whereby a cockroach continuously updates its knowledge base on the direction and distance to a new feature in their harbourage arena2, 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.

NUTRIENT AND ASSOCIATIVE LEARNING

Cockroaches also 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.

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.

INFORMATION SHARING

Once a quality food source is located, they will share information with other cockroaches in close proximity. This grouping mechanism leads to the formation of feeding aggregations and the occurrence of collective food selection. A study by Lihoreau and Rivault3 leads to the conclusion this is due to olfactory cues arising from their feeding nest-mates, suggesting the cue is a yet to be identified foraging pheromone.

Mathematical modelling suggests group foraging may be a less than optimal strategy when large numbers of cockroaches are present. However, there may be benefits to this behaviour since group feeding offers an anti-predation strategy, increasing an individual’s chance of escaping threats. Also, since aggregates usually consist of different developmental stages, there may be benefits for nymphs.

Their foraging activity and behaviour are determined by the age of the cockroach.

Cockroaches at different stadia forage differently. Early stadia nymphs forage in a similar manner throughout that stadium, whilst fifth stadium nymphs start to show path integration and evolve more efficient foraging behaviour4.

COCKROACH MANAGEMENT

First and second stadia nymphs rarely leave their harbourages, travelling further as they mature until, by the fifth stadium, they are highly active in their foraging. Naturally occurring populations of cockroaches show a relatively constant ratio of nymphs in the total population, of 0.6. This is largely unaffected by the size of the population and is expressed as the nymph-to-total ratio (NTR). Given this high ratio of nymphs, it is important that we target them in baiting programmes.

Durier and Rivault5, 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 which, in this study was 600mm away, but near a visual landmark known to the cockroaches.

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, the location of baits is important, regardless of the palatability of the bait.

The logical conclusion from this is that baits should be placed as close as possible to cockroach harbourages; with various studies6 demonstrating that gel baits applied in small drops at multiple locations provide greater efficacy than simply placing a few large drops or smears of bait.

Placement of multiple bait spots also avoids aggressive behaviour among cockroaches. Dominant cockroaches will aggressively protect a favoured food source, attacking early life-stage cockroaches or less vigorous individuals to defend their food source. While these dominant individuals will later die from the bait toxin; those cockroaches that were chased away, are unlikely to return, as they will have integrated that experience into their knowledge base and stay clear of the area in the future.

In conclusion, baiting programs can be best enhanced by using monitor traps to identify where the cockroach harbourages are, before placing multiple bait spots near these identified locations. The key aspect of 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. 

This is part one of two articles on cockroaches by Steve.
Look out for ‘Creating excellence in cockroach baiting programmes’ in the next issue of PPC.

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Steve will be talking about cockroach foraging behaviour at PestEx 2019. Register for PestEx for free now.
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[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.
[2] Collett, T.S. and Graham, P. (2004) Animal navigation: path integration, visual landmarks and cognitive map. Current Biology 14, R475-R477.
[3] Lihoreau M., Rivault C. (2011) Local enhancement promotes cockroach feeding aggregations. PLoS ONE 6(7): e22048. doi:10.1371/journal.pone.0022048
[4] 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.
[5] 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.
[6] 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.

Source: PPC94