Final Comprehensive Conservation Plan - U.S. Fish and Wildlife ...

Hakalau Forest National Wildlife Refuge
Comprehensive Conservation Plan
Mosquito distributions are also affected by ambient temperature, with population growth rates
increasing with temperature. Ahumada et al. (2004) estimated that mosquito populations on Hawai„i
Island can survive in areas with an annual temperature of 58.3°F and a summer temperature of
55.8°F. LaPointe (2000) found that the southern house mosquito can complete larval development at
53.6°F. As a result of these temperature restrictions, mosquitoes on the Island of Hawai„i can survive
up to approximately 4,839 ft. This range could extend to 5,625 ft during the summer. Rainfall also
influences mosquito prevalence and survival because adults require water filled cavities to breed and
larval stages are highly susceptible to drought (Ahumada et al. 2004). Additional factors, such as
size, can contribute to survival rate. Larger individuals have a higher feeding success, survivorship,
and infection potential (LaPointe 2000).
The southern house mosquito is a primary vector for several diseases that impact native Hawaiian
birds. Mosquito-borne avian diseases, principally avian malaria and the avian pox, have been
implicated as the main reason for mortality of the native Hawaiian forest birds (Van Riper et al.
2002, LaPointe et al. 2005, Reiter and LaPoint 2007). As a result, some bird species are only able to
survive at higher elevations, above the mosquito zone. Other mosquito species have been shown to
carry both diseases to a lesser degree (LaPointe et al. 2005).
Although mosquitoes are able to persist and complete their life cycle at higher elevations, avian
malaria oocysts may not be able to develop in cooler temperatures. Benning et al. (2002) estimated
that the threshold temperature for transmission of avian malaria is 55.4°F. Avian malaria prevalence
is highest in mid-elevation forests with annual temperatures of 63°F. Thus, mosquitoes living at
higher elevations may not be able to transmit avian malaria (Ahumada et al. 2004).
Western Yellowjacket Wasps (Vespula pensylvanica)
Western yellowjacket wasps were first recorded in the Hawaiian Islands in 1919, although they did
not become established on Hawai„i Island until the 1970s. The annual cycle of the western
yellowjacket wasps is regulated by climate (Gambino and Loope 1992). The queen, which hibernates
during the winter months, establishes a colony in spring, and populations subsequently peak in the
summer. However, because of Hawai„i‟s warmer climate, overwintering colonies occur irregularly in
the Hawaiian Islands (Gambino and Loope 1992, Nishida and Evenhuis 2000, Haines and Foote
2005).
This wasp species threatens native insect communities by preying on native species, especially larvae
and pupae. This includes rare and endangered species such as Drosophila flies (Mitchell et al. 2005)
and the koa bug (Johnson et al. 2005). In addition, the wasps compete with native predators and
pollinators for various food resources. This may in turn have a larger ecosystem impact by removing
prey for native species (Haines and Foote 2005). It has been suggested, but there is no evidence, that
the wasps may feed on native birds or their eggs and nestlings (Jeffrey, pers. comm.).
Preliminary work on the distribution of western yellowjacket wasps at the HFU has been conducted
(Foote 2002, Howarth et al. 2003). The prevalence of these wasps on the HFU is cyclic (USFWS
unpubl.). Trapping during the early 1990s shows that during wet years, few western yellowjacket
wasps are observed, and populations increase during dry years (Jeffrey, pers. comm.).
At the KFU, populations of western yellowjacket wasps have been monitored using plastic
yellowjacket traps hung from branches and baited with heptyl butyrate (Haines and Foote 2005). The
Chapter 4. Refuge Biology and Habitats 4-95