Studies on Anopheles Walkeri Theobald Conducted at Reelfoot Lake, Tennessee, 1935–1941

IX. Summary and Conclusions

1. 1.Studies of the bionomics of Anopheles walkeri Theobald were made during the summers of 1935 to 1941 at Reelfoot Lake, Tennessee. A. walkeri is a species of wide distribution throughout eastern North America, but is abundant only in certain rather restricted areas. At Reelfoot Lake, light-trap collections of adults over a period of five years showed that this species formed about 38.5% of the total anopheline population.
2. 2.At Reelfoot Lake, Anopheles walkeri breeds in thick, shaded emergent vegetation. The larvae were most frequently found among cut-grass sheltered by willows or button-bushes. However, with changing water-levels, breeding occurred in other situations, such as partly submerged beds of smartweed, skullcap, water-hoarhound, and hemp-vine.
3. 3.Observations on dispersal showed that Anopheles walkeri could be taken in light-traps at distances of 1½ and 2 miles from the nearest breeding places.
4. 4.Anopheles walkeri were found resting during the daytime in dark, extremely moist situations. The favorite resting site was upon the dark shaded bases of mature cut-grass, but if deprived of shelter here they could be found in the dark sanctuary of the shoreline button-bushes and the flooded forest. They were usually found resting a few inches above the water surface, only occasionally above moist mud, and very infrequently in drier places. Females engorged with fresh blood were found resting temporarily near the moist floor of barns and occasionally in human dwellings.
5. 5.Under experimental conditions, the number of Anopheles walkeri penetrating into houses depended greatly upon the illumination and appeared to vary roughly with the intensity of light at the point of entrance. The presence or absence of human beings within the dwelling did not appear to have much influence. Under natural conditions, Anopheles walkeri was found in dwellings much less frequently than Anopheles quadrimaculatus, and then only in poorly screened houses with some degree of illumination.
6. 6.A single specimen of 2,003 Anopheles walkeri was found infected with an unidentified species of Plasmodium. Three of 1,171 Anopheles quadrimaculatus examined at the same time were infected with plasmodia. Further evidence will be required to incriminate Anopheles walkeri as a malaria vector of real menace to human populations, although many of its habits point definitely in this direction.
7. 7.Specific differences in halter coloration, scales of palpi and proboscis, and salivary glands were found to exist between Anopheles walkeri and the closely related Anopheles quadrimaculatus, in addition to the other distinguishing characters that are already common knowledge.
8. 8.The so-called “winter egg” of Anopheles walkeri was laid at Reelfoot Lake during September 1–15, 1937 and September 16–23, 1939. Other observations on the physiology of the species showed that it was more susceptible to heat than Anopheles quadrimaculatus, but both species showed increased resistance when fed on sugar solution or blood. Light-trap collections showed that large numbers of Anopheles walkeri were in flight only at temperatures above 75°, with one unaccountable exception.

Finally, let us consider the bionomics of Anopheles walkeri in relation to the transmission of human malaria. This species is found in parts of the United States where malaria exists in endemic form, such as the southeastern states and the lower Mississippi valley. Perhaps of equal significance is its presence in northern regions from which the tides of malaria have ebbed, but where the brief, devastating epidemic is still a menace.

Anopheles walkeri exhibits a fastidious discrimination with regard to breeding places, but multiplies bountifully where favorable ones exist. The adults disperse for at least two miles in quest of food. They bite fiercely, even in bright daylight if near their moist resting places, and feed on man and domestic animals with equal avidity where both are available. The species appears to be considerably more phototropic than A. quadrimaculatus and its entrance into houses may be facilitated by the introduction of modern electric lighting into rural areas. Lastly, Anopheles walkeri has been shown to be a host of at least two species of human malarial parasite.

On the other hand, it frequently bites the more available domestic animals and does not enter dwellings as readily as A. quadrimaculatus. From the observations on the physiology, one sees that it is a delicate species and cannot long survive the conditions imposed upon most mosquitoes who choose to enter houses—and attempt to leave. Also to our knowledge, malaria is not yet known to exist where this species is the only anopheline. In areas of endemic malaria in this country where both A. walkeri and A. quadrimaculatus are found, the latter is apparently the important vector.

We have seen then that the effectiveness of Anopheles walkeri as a potential vector of human malaria depends on the maintenance of an exceptionally favorable environment and ample opportunity for contact with humans. These conditions are fulfilled at Reelfoot Lake. Where they have developed in regions formerly free from malaria, the introduction of the parasite by present shifting populations may reveal Anopheles walkeri as a vector of importance.