Blaberus discoidalis has been found in homes or in fruit debris in Puerto Rico in company with the more common, domiciliary species Leucophaea maderae, but never in abundance (Sein, 1923; Wolcott, 1950). Illingworth (1915) in Hawaii found Symploce hospes associated with Nauphoeta cinerea, Graptoblatta notulata, and Diploptera punctata.
Hebard (1917) found Aglaopteryx diaphana in a bromeliad on a forest tree in Jamaica together with Nyctibora laevigata and numerous Cariblatta insularis. He also found numerous Aglaopteryx gemma under signs on longleaf pines in Alabama with occasional specimens of Parcoblatta lata. In Virginia he found Parcoblatta uhleriana in a decaying chestnut log with Cryptocercus punctulalus. In Florida he found Latiblattella rehni with Eurycotis floridana and, more rarely, with Periplaneta australasiae under bark of pine trees. In Key West he found Symploce hospes in the cupboard of a hotel with swarms of Blattella germanica and a few Supella supellectilium.
Rehn and Hebard (1927) in their study of West Indian blattids reported finding Neoblattella proserpina in epiphytic bromeliads in Jamaica in company with Neoblattella eurydice and Neoblattella dryas. They also list most of the associations cited by Hebard (1917).
Ramme (1923) reported that he found in Germany four species of Ectobius (lapponicus, lucidus, pallidus, and sylvester) living together in an area about 50 m. by 200 m. Although he had stated that his specimens of E. lucidus were a distinct species in 1923, Ramme (1951) later decided that they were a form of E. sylvester, E. sylvester f. lucidus.
Dow (1955) reported trapping Blattella germanica, Periplaneta americana, and Periplaneta brunnea in houses and privies in south Texas. At our request Dr. Dow (personal communication, 1958) analyzed his records to determine whether there were indications of associations between these species, with the following results:
As stated in my published note, the roaches were at first classified to genus only. The 83 Periplaneta subsequently identified to species represented 28 different collections, 11 from houses and 17 from privies, all in Pharr, Texas. Tabulation of the data shows first that P. americana was taken only once in a house and that P. brunnea was taken only 4 times in privies. Of course this distribution greatly reduces the probability that they would be caught together, and it is not surprising that P. americana was trapped alone in the single house collection. P. brunnea, however, was trapped with P. americana 2 of the 4 times it occurred in privy collections.
To investigate the occurrence of Periplaneta with and without Blattella, an analysis has been made of 560 trap collections taken in 40 houses and 40 associated privies in Pharr, Texas, in weekly intervals (from May 14 to June 23 [1948]). In the houses, Periplaneta and Blattella were caught in the same jar 26 times, Periplaneta alone 12 times, Blattella alone 83 times, and neither genus 159 times. In a fourfold table, the value of chi-square (14.7) is significant and indicates that the frequencies are not proportional. The number of times Periplaneta and Blattella actually occurred together (26) is, however, much larger than the expected number calculated from the row and column frequencies (14.8). In the privies, Periplaneta and Blattella were caught in the same jar 9 times, Periplaneta alone 50 times, Blattella alone 18 times, and neither genus 203 times. In a fourfold table, the value of chi-square (1.95) is not significant but the same type of disproportion is evident and the expected frequency of both genera in one trap is 5.7, lower than the actual frequency of 9. Both immature and adult roaches are included in this analysis.
The above evidence would be more satisfactory if based on more extensive data. There is also a possible objection in that the traps were operated for at least overnight, during which time one species could theoretically supplant another. Of course, it is doubtful that there is anything involved here like territory (in the ornithologists' sense). On the other hand, it is well to consider that Periplaneta and Blattella are both likely to be more abundant in the same type of favorable location and that this factor might offset in part some direct antagonism between the species.
The only known specimen of Ischnoptera podoces was captured in company with the type series of Cariblatta nebulicola, in dead leaf litter in Jamaica (Rehn and Hebard, 1927). In Florida Periplaneta australasiae was often taken in company with Pycnoscelus surinamensis and Eurycotis floridana (Blatchley, 1920).
INTERSPECIES ANTAGONISM
In contrast to the presumably amicable associations mentioned above, other observations in the literature seem to indicate that some species of cockroaches are incompatible when they attempt to occupy the same habitat niche. Marlatt (1915) stated "Rarely do two of the domestic species occur together in the same house. Often, also, of two neighboring districts one may be infested with one species, while in the other a distinct species is the commoner one. The different species are thus seemingly somewhat antagonistic, and it is even supposed that they may prey upon one another, the less numerous species being often driven out." Phelps (1924) stated "Roaches of different species are rarely found together, although roaches of the same species live together on very amicable terms."
In 1859 Darwin (1887) stated that "In Russia the small Asiatic cockroach [Blattella germanica?] has everywhere driven before it its great congener [Blatta orientalis?]." Yet in France Girard (1877) suggested that the oriental cockroach be introduced into a restaurant infested with the German cockroach as the best way to expel the latter, because the more robust species drives away cockroaches of smaller size. Wille (1920) in Germany found usually only one species of cockroach in a house. Yet when he placed B. orientalis and B. germanica together, there were no reciprocal attacks even by hungry individuals. Wille concluded that because of their greater speed, smaller size, greater number of eggs, and faster development, the German cockroaches eat the available food and so make the environment unfavorable for the oriental. However, he noted that cases may be seen in which the opposite is also possible. Laing (1946; British Museum [Natural History], 1951) observed that in the British Isles B. orientalis seems to have lost its dominant position to B. germanica in recent years; it was stated that these species are not as a rule found together and that the greater rapidity of breeding and ability to climb of B. germanica, as well as the layout of modern buildings, are some of the factors that favor the spread of B. germanica. Ledoux (1945) found that first-instar nymphs of B. germanica and fourth-instar nymphs of B. orientalis, adults of B. germanica and sixth-instar nymphs of B. orientalis, as well as adults of both species, did not form mixed groups. However, when he combined fifth-and sixth-instar nymphs of B. germanica with fourth-and fifth-instar nymphs of B. orientalis, which are all practically of equal size, sometimes he would find mixed groups, but generally the groups were distinct. Lucas (1912) stated that Burr had found B. germanica and B. orientalis swarming within a rubbish heap in England; presumably both colonies were breeding and multiplying and one species was not detrimental to the presence of the other.