THERMAL ECOLOGY OF A MALARIAL PARASITE AND ITS INSECT VECTOR - CONSEQUENCES FOR THE PARASITES TRANSMISSION SUCCESS

1. We examined the transmission biology of Plasmodium mexicanum, a par asite of the fence lizard Sceloporus occidentalis, and its vector, the sandfly Lutzomyia vexator. Female L. vexator produced a dutch of eggs after each blood meal taken from a lizard. Mortality was high after o viposition, so few sandflies were likely to take two blood meals and a lmost none took three. Therefore, to maximize its transmission success , the parasite must complete development in its insect host before the vector lays its eggs and takes another blood meal. 2. Between 16 degr ees C and 32 degrees C, temperature did not affect the longevity of fe male sandflies, but did affect the rate of parasite development in the insect, the rate of maturation of sandflies' eggs, and the probabilit y of sandflies becoming infected. 3. The above relationships with temp erature were non-linear and differed in shape among the variables such that an increase in temperature between 22 degrees C and 32 degrees C benefited the parasite by shortening its development while not reduci ng the time until the sandfly's next blood meal. 4. We measured the te mperatures available to the vectors in nature (burrows of ground squir rels). Within this range, there was a window that allowed successful t ransmission of the parasite (based on laboratory studies). 5. In a the rmal gradient, unfed female sandflies selected mean temperatures appro ximately 4 degrees C below the minimum required for transmission. Afte r a blood meal from a non-infected lizard, the insect's mean preferred temperature increased 1.6 degrees C, presumably to aid digestion, and ifa blood meal was taken from an infected lizard mean preferred body temperature increased by 3.6 degrees C. 6. Compared with 10 other Plas modium species, P. mexicanum has a very rapid rate of development in i ts vector. 7. The results suggest P. mexicanum enhances its transmissi on success through a combination of rapid development in the insect ho st and manipulation of the vector's thermoregulatory behaviour.