Entomol. exp. appl. 61: 149-158, 1991. �9 1991 Kluwer Academic Pubhshers. Printed in Belgium 149

Exploitation of food resources by the cockroach Blattella germanica in an urban habitat

C. Rivault & A. Cloarec Laboratoire d'Ethologie, CNRS URA 373, Universitd de Rennes I, Campus de Beaulieu. 35042 Rennes cedex, France

Accepted: June 3, 1991

Key words: Cockroach, foraging, ideal free distribution, dynamics of patch exploitation

Abstract

The exploitation of food resources by the German cockroach, Blattella germanica (L.) (Dictyoptera: Blattellidae) was investigated experimentally in relation to distance from shelters and depletion of neighbouring food patches. In addition, the dynamics of exploitation of a patch were analysed. Obser- vations were made after dark in a public swimming baths building and each one lasted 3 h. Food patches were placed in rows, at different distances from the shelters. The number of cockroaches in food dishes, in a 20 cm diameter circle round each food dish and in a 60 cm diameter circle round this first circle were recorded. Food items nearest the shelters were exploited first. Exploitation of row 2 and of row 3 food items started later, after row 1 food patches had been depleted. Under these conditions, the moment a food patch was exploited was related to its distance from shelter. Exploitation of food patches occurred in a step-by-step manner, one patch attracting animals when a nearby patch had been depleted, and not following a model of ideal free distribution.

Although our experimental food patches were exploited in relation to their distance from shelter, we were able to demonstrate that distance did not influence the dynamics of exploitation of a food item. The mean number of cockroaches on a food patch, whatever its spatial position, increased regularly, reached a maximum at t = - 10 min, and then decreased rapidly after all the food had been completely consumed, at t = 0 min. The mean number of animals in the 20 cm diameter circle round a food source peaked at t = 0 rain, then decreased rapidly. This area appeared to be a transit area. The mean num- ber of animals in a 60 cm diameter circle round the food source peaked later, and then decreased slowly. Animals remained in this area longer than in the area closer to the food dish, but their presence there was concomitant with the depletion of the food box.

Introduction

The German cockroach, Blattella germanica (L.) (Dictyoptera: Blattellidae), is a species that is found in urban habitats, particularly in restau-

rants, bakeries, kitchens etc., where appropriate food resources can be considered to be abundant and easily available for this omnivorous species.

A long term study of the dynamics of a popu- lation ofBlattella germanica in a public swimming-

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baths showed that the level of this population did not vary significantly in spite of favourable envi- ronmental conditions (hygrometry, temperature, shelters...) (Rivault, 1989). The only possible lim- iting factor appeared to be food. Food depriva- tion under laboratory conditions has been shown to block reproduction (Durbin & Cochran, 1985) or moulting in larvae (Kunkel, 1966). This cock- roach population seemed to depend on cake and biscuit crumbs, remains of sweets or fruit, etc. dropped randomly by the public around the pools. Food, therefore, was typically distributed in patches that varied temporally and spatially.

There is an almost complete lack of informa- tion in the literature on data concerning foraging by cockroaches under natural urban conditions. When food availability varies temporally and spa- tially, the average rate and quantity of food intake may depend on the search method used. Some of the basic foraging models concern choice and ex- ploitation of food patches. These models present assumptions about compromises made by an or- ganism to evaluate patch residence time, taking into account various constraints like interpatch travel time (Krebs etal., 1978; Kacelnik, 1979, 1984; McNair, 1982; Kamil & Roitblat, 1985; Stephens & Krebs, 1986). In the present case, these foraging models were used as a tool for understanding the organization of this urban population.

The aim of this investigation was to study how cockroaches, from this natural population, ex- ploited food resources under these rather limiting conditions. We analysed: a)how cockroaches were distributed between food items in relation to the distance of food items from the shelters and to the depletion of neighbouring food items, and b) the dynamics of the exploitation of one food item from the moment the first animal discovered it, until the food was completely consumed and all the foragers had left the area. The question ad- dressed here was: would late comers at the nearer food sources prefer to forage on these already reduced food sources where they would have to face competition, or would they prefer to forage further away from the shelters on patches where there was less competition?

Material and methods

Experimental area The observations were carried out in a public swimming-baths building. Hygrometry, tempera- ture and photoperiod remained approximately constant. The building was open to the public until 22h00 when all the lights were switched off. Lights were switched on again next morning at 06h00 when the employees came to clean the baths.

Experimental set up Fourteen standard food sources were placed on a 4 m wide tiled poolside which stretched between a tiled bench placed all along a wall and the pool. In this part of the building cockroaches sheltered behind the central heating grids opening under the bench. Fourteen food sources were placed along 3 rows, parallel to the bench, 1.25 m apart. Row 1 was 0.75 m from the bench, row 2, 2.0 m and row 3, 3.25 m (Fig. 1). The number of food sources increased with distance from the shelters. There were 2 food sources in row 1, 5 in row 2 and 7 in row 3. This arrangement was chosen so that greater shelter-patch distances were counter- balanced by higher patch densities. In addition, interpatch distances were relatively short.

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Test food sources of 0.05 g dry bread were wet- ted with 3 drops of beer just before an observa- tion. The wetted bread was placed in a small transparent plastic dish, 3 cm in diameter with an edge 0.5 cm high. Even the smallest cockroaches had easy access to the food.

A 4.5 volt battery light was fixed approximately 0.80 cm above each food source to facilitate ob- servations. Preliminary observations showed that this continuous dim light neither disturbed the animals nor modified their behaviour, whereas an intermittent light, switched on just before an observation made the cockroaches flee. This ex- perimental disposition was set up before an observation, before the lights were switched off.

Observations Each observation lasted 3 hours and started when the lights were switched off, around 22h00. Scan records were carried out every 5 min on all of the 14 food sources. Each record included: a)The total number of animals on the food source; as it was impossible to verify if each cockroach was really eating, it was assumed all the animals in the dish (7 cm 2) had found the food source: b) The number of animals outside the food dish, but within a circle 20 cm in diameter centered round the food dish. This circle, named circle A, cov- ered approximately the surface lit by the battery (307 cm2); c) The number of animals outside the previous circle but within a circle 60 cm in diam- eter (2513 cm2), named circle B.

Thirteen 3 h observations were carried out over a period of 3 weeks.

Recentering data Time taken to deplete each food source varied greatly. Therefore, we chose as new reference time, the time a food source was entirely con- sumed. Negative times then indicated records when food was still present in the dish, zero in- dicated the time the dish was emptied and posi- tive times indicated records made after the deple- tion of a dish.

Only data for completely depleted food dishes were taken into account.

In order to analyse in detail the distribution of

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cockroaches between patches in relation to de- pletion of a food dish:

a) the numbers of animals recorded on row 1, 2, and 3 food sources were evaluated in related to the moment each row 1 food dish was depleted;

b) the numbers of animals recorded on row 2 and 3 food sources were evaluated in relation to the moment each row 2 food dish was depleted.

In order to analyze in detail the dynamics of complete depletion of a food patch by cock- roaches in relation to distance from shelters, re- centered data for depleted dishes were pooled row by row.

All food sources in row 1 were completely con- sumed on each observation day (26 sources = 2 dishes x 13 observation days). 25 out of a possi- ble total of 65 ( = 5 dishes x 13 observation days) row 2 food sources were completely consumed, and finally 16 out of a possible total of 91 ( = 7 dishes x 13 observation days) row 3 sources were completely consumed.

Results

Distribution between patches in relation to distance from shelters

Cockroaches generally started coming out of their shelters and exploring the poolside as soon as the lights were switched off. Although each food source was observed during the entire 3 h period, an important variability was recorded between sources and from one observation day to another. The quantity of food consumed on each food source varied between dishes during one obser- vation and, for a given dish, it varied between observations. The number of animals on and round row 1 food sources increased very rapidly in relation to time and reached a maximum ap- proximately 50 to 60 min after the beginning of the observations, then decreased gradually as the cockroaches moved further away from the shel- ters.

Very few animals were observed on row 2 or on row 3 food sources before row 1 food sources had been completely consumed. After the first obser-

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vation hour the numbers of animals on row 2, then on row 3, increased more rapidly. The num- ber of animals round row 2 and row 3 dishes reached a maximum during the last observation hour, much later than on row 1, and later on row 3 than on row 2.

Data centered on depletion of row 1 food dishes characterized mean exploitation of row 1 food sources in relation to depletion instead of first arrival (Fig. 2). The mean number of animals on a row 1 food source increased gradually at first, then faster and reached a maximum just before the food was completely depleted. When there was no food left, all the animals dispersed rapidly.

Data for all row 2 food dishes, and for all 7 row 3 food dishes were recentered taking the time row 1 food dishes were depleted as reference time.

Very few animals were found on row 2 and row 3 food sources before row 1 food sources had been completely eaten. In addition, data centered on depletion of row 2 food sources showed that ex- ploitation of row 2 food sources was similar to that of row 1 food sources and preceded exploi- tation of row 3 (Fig. 3).

These data showed that the exploitation of a food source in relation to time from the beginning of an observation was related to its position in the

environment. Row 1 food sources, near the shel- ters, were exploited first; then, when these were depleted, animals moved further from the shelters and foraged on row 2 food sources, and even later they moved from row 2 to row 3.

Therefore the exploitation of a food source in relation to time since the beginning of an obser- vation was related to its distance from shelters.

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Dynamics of exploitation of a food patch

The dynamics of the exploitation of a food source were analysed by taking into account the number of animals recorded at different distances from a food source in relation to depletion time. Three different areas were observed for each patch (see material and methods): the food dish, circle A round the food dish and circle B round circle A.

Exploitation of food sources near shelters

Dynamics of exploitation of food source in the food dish.. All data for row 1 food dishes (Fig. 4) were pooled and the mean number of animals in the food dish remained low (<2) until t= -60 min, then increased rapidly and regularly and reached a maximum (between 9 and 10 animals) between times t = - 10 min and t = -5 min. All row 1 food

sources were completely consumed in approxi- mately 60 min. At time t = 0 rain, i.e. the first record following total depletion, many cock- roaches had already left, and there were only ap- proximately four subjects still in the dish. That means that less than 5 min after all the food had been consumed over 50~o of the foraging animals present in the dish had left. Ten minutes later (at t = + 10 min), only two subjects on average could be found in the dish. The depletion of a food source induced rapid departure of animals that had been feeding at that source.

Variations of population density in circle A. . The mean number of animals in the immediate vicinity of a food dish (circle A) (Fig. 4) did not exceed 2 before t = -15 rain, peaked between t = -5 min and t = + 5 min, then decreased rapidly. A drop of over 50~o had already occurred at t = + 15 min.

154

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Fig. 4. Dynamics of exploitation of a row 1 food source. Mean number of cockroaches in the food dtsh, in circle A and m ctrcte B, in relation to time, in mm. All data were recentered on depletion of row 1 food dishes. Legend: t: 0 min: depletion time; dash: + 1 S.D.

Cockroaches were found in this area mainly just before and just after depletion, but they never stayed very long close to the food dish, either before or after t = 0 min. This area appeared to be a real transit zone only crossed by animals to reach the food source or to leave it. They could probably perceive the source from outside cir- cle A and therefore they went straight to the food. There were no animals in this area waiting to gain access to the food dish. In addition, they showed no tendency to remain in the immediate vicinity of a depleted dish.

Variations of population density in circle B. . The mean number of animals in row 1 circle B (Fig. 4) did not exceed 2 before time t = -40 min, then increased gradually and reached a maximum (ap- proximately 10 subjects) between t -- + 5 min and t = + 15 rain. The number of animals in circle B

then decreased very slowly. It took over 30 min, until t = + 40 min, for the number of animals in circle B to drop 50~'o. That means that the de- pletion of the food dish did not induce an imme- diate departure from this area.

The number of animals started to increase in the food dish first, and then only in the surround- ing areas. After all the food had been eaten, cock- roaches left the food dish area and circle A very rapidly, but many remained in circle B for over half an hour.

Exploitation of food sources further from shelters. We showed above that row'2 and row 3 food sources were exploited much later than row 1 food sources. In spite of that, the data analysis method used here showed that the exploitation dynamics of a row 2 and row 3 food sources appeared very similar to those for row 1.

General pattern of exploitation of a food source

Data for food sources for the three rows pre- sented very similar dynamics. Therefore the mean numbers of animals recorded in the food dishes were compared between rows for each time in- terval. These comparisons revealed no significant differences between the data for the three rows (T-tests, P > 0.01 in all cases). The mean numbers of animals in circles A and B were also compared between rows. Except for one or two cases due to small sample size, no significant differences were found (T-test, P > 0.01).

Therefore the three sets of data (presence of animals in the food dish, in circle A and in cir- cle B were pooled for all the food sources that had been completely eaten, whether they were placed on row 1, 2 or 3. Thus, we were able to charac- terize in detail the dynamics of exploitation of a food source (Fig. 5).

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The total number of animals for the three areas (in and round the food dish) reached a maximum at t = -5 min, but comparisons between the sets of data for the three different areas revealed dif- ferences between the time when the mean num- ber of animals peaked: at t = - 10 rain in the food dishes; at t = 0 min in circle A; at t ; + 5 min in circle B. As long as there was some food in the dish, the number of animals there increased reg- ularly, but as soon as all the food in the dish had been eaten animals started to leave and they left the food dish very rapidly. It must be stressed here that between t= -15 min and t= -5 min there were very many animals in the food dish although there was already very little left to eat.

The presence of animals in circle A was only transitory and corresponded mainly to the mo- ment when animals left the food dish they had just emptied. The presence of animals in the larger circle B lasted longer, but was concomitant with

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the depletion of the food dish. Increases in num- bers in circle B were similar to those in food dishes but occurred later. However declines differed, they were rapid in food dishes but more gradual in circle B. As long as there were only a few an- imals that had found the food, they all had easy and direct access to it. They did not need to stay long in its vicinity, that is in either of the two circles. When the number of animals in the food dish became more important, the number of an- imals in circles A and B started to increase. How- ever, a mean value of 7 animals in the food dish must be reached before a mean of 1 animal was recorded in circle A or a mean of two animals was observed in circle B.

Discussion

A hungry cockroach has several kinds of deci- sions to make, mainly, to choose where to forage, and how to exploit the food it finds in relation to the number of other cockroaches present. Blattella germanica is truly omnivorous and eats just about everything it finds in the urban habitat where we studied it (Cornwell, 1976; Cochran, 1982). Cockroaches use olfactory cues to find their food, they forage individually, but often converge on the same places.

The data presented above indicated that cock- roaches did not forage following an ideal free dis- tribution (Stephens & Krebs, 1986). Food patches placed near the shelters were depleted before patches placed further away. Cockroaches stopped at the first food patch they encountered. Not only the food sources nearest the shelters were completely consumed first, but they were always completely eaten before animals were ob- served going toward food sources further away, although access to the first food source became gradually more difficult as the number of cock- roaches round it increased, and its size decreased.

The shorter the distance between shelters and food sources, the higher the probability a food source will be found rapidly, and exploited rap- idly. Food sources appeared to be exploited in a step by step manner, animals moving away only

after depletion of the nearest food patches. Ob- servations indicated that some animals at least went from a depleted dish to a neighbouring one, but the proportion of animals feeding successively on several sources remains to be evaluated. We suggest that this strategy does not agree with ideal free distribution foraging models because the risk of being predated or of being lost in an unknown environment when foraging far from the usual shelter is higher than the risk of being deprived of food.

The main point revealed by the observations reported here shows that the dynamics of exploi- tation of a food patch follow the same pattern whatever the position of the patch in the environ- ment and therefore a general pattern representing the exploitation of our experimental food sources by this urban population could be described (Fig. 5). There appears no need for complex rules at the individual level for complex behaviour to emerge at the group level. The influence of the environment can be just as important as individ- ual behaviour on the emergence of collective fac- tors (Deneubourg et al., 1989).

The dynamics of recruitment of animals such as ants to a new food source have been described by a logistic type equation (Pasteels et al., 1987). Buser et al. (1987) also modelled recruitment ef- ficiency. The number of ants at a fixed bait in- creased gradually and regularly after the first food discovery and reached a peak approximately 70 rain later. This can be compared to the general trend observed in cockroaches.

This exploitation strategy raises problems con- cerning access to a source, sharing the food and when to leave a patch.

Cockroaches live in large aggregates (Rivault, 1989). Many adults and larvae can gather in the same shelter. They disperse from the shelter over great distances to find resources. This could be referred to as the refuging system as defined by Hamilton and Watt (1970) where interindividual aggressive encounters, when they occur within the foraging area, are generally confined to the immediate vicinity of the individual. Encounters occur at close range, within the context of a tem- porary but cohesive social unit. However some

aspects of central place foraging models (Orians & Pearson, 1979), although they apply to animals which carry food towards a central place, could apply in this case because these models stress the fact that animals should forage in the closest patch exclusively. It appears that cockroaches choose to consume food sources placed near shelters en- tirely, in spite of interference from conspecifics, before searching for a new food patch.

Benefits are assumed to increase at a decreas- ing rate with group size (Giraldeau, 1987), but then a selfish animal is expected to join a group so long as it can receive some benefit from doing so (Sibly, 1983; Clark & Mangel, 1984). Distance travelled and therefore energy output, appeared to be an important factor although the food sources placed the furthest from the shelters were well within the nightly range of a cockroach (Fig. 1).

The number of animals round the food sources near the shelters was very important at some times. A similar phenomenon has been described in spiders (Uetz, 1988; Rypstra, 1989). Spiders gathered in colonies and their density increased rapidly in areas of high prey density, but they dispersed just as rapidly when prey density de- creased. This phenomenon implies an important interindividual tolerance in the spiders which is also found in cockroaches. The intrasocial com- petition that develops in ants on overcrowded food sources induces the end of trial reinforcing (Pasteels et al., 1987). Similar competition be- tween cockroaches did not have such an acute effect: only the complete depletion of a food dish induced departure.

Acknowledgements

We thank Mr G. Secretain, Director of the swimming-baths, and staff members for all the facilities and comprehension during the course of the project. The project was supported by an ATP PIREN-Environnement: 'Ecologic des Invasions associ6es aux perturbations de l'environnement'.

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