Vapor Pressure Deficit Differentially Affects Laboratory Populations of Metaseiulus occidentalis and Neoseiulus fallacis (Acarina: Phytoseiidae) Reared Together

Abstract

The phytoseiid predators Neoseiulus fallacis (Garman) and Metaseiulus occidentalis (Nesbitt) were reared together and fed on Tetranychus urticae Koch to determine the possibility of coexistence and the effect of vapor pressure deficit on the two species. Additionally, developmental times for the two predators feeding on the spruce spider mite, Oligonychus ununguis (Jacobi), were determined at 14°C and vapor pressure deficits of 1, 4, 6, and 10 millibars (mb). N. fallacis and M. occidentalis respond to vapor pressure deficit in essentially opposite ways. In the interaction treatments, populations of N. fallacis reproduced rapidly at low vapor pressure deficits (high atmospheric moisture) but vapor pressure deficits of 10 mb or greater severely limited population growth of this species; M. occidentalis populations grew more readily at higher vapor pressure deficits (>10 mb). At 14°C, egg mortality for M. occidentalis was 25, 10, 20, and 45% at 1, 4, 6, and 10 mb, respectively; egg mortality for N. fallacis at these respective vapor pressure deficits was 25, 0, 15, and 80%. At 14°C, developmental periods for immature stages of N. fallacis among vapor pressure deficits were significantly different in the egg and larval stages and in the egg stage for M. occidentalis . N. fallacis is directly subject to the physical environment (vapor pressure deficit effect on egg mortality) whereas M. occidentalis is subject to differences in survival responses (less prolific than N. fallacis at a given vapor pressure deficit and temperature). The differential response of the two predators suggests that releases of both species to control O. ununguis on Fraser fir in western North Carolina can be optimally timed based on humidity conditions.