Hemlock woolly adelgid biology
There are two generations of hemlock woolly adelgid (HWA) per year on hemlock - the spring progrediens generation (March-June) and the overwintering sistens generation (June-March). The progrediens generation has two forms - a wingless morph that stays on hemlock, and a winged morph (sexupurae) which flies in search of a suitable spruce host. In their native range, sexupurae reproduce sexually on spruce, however in North America there are no known suitable spruce species, so these winged insects are thought to be unsuccessful. All HWA on hemlock, both sistens and progrediens, are female and reproduce asexually.
Both progrediens and sistens follow the same developmental stages. Eggs hatch to crawlers (young nymphs) which are the dispersing stage of the insect. Crawlers search for suitable feeding sites at the base of hemlock needles. Once a nymph inserts its mouthparts in to a twig to feed, it becomes immobile. Adults stay in situ, producing a characteristic woolly ovisac which covers its eggs. The ovisac is a waxy material, which likely provides protection to the developing eggs.
The progrediens generation is brief, sistens adults lay 50-175 (up to a maximum of 300) eggs starting March-April until early May. Depending on temperature, these progrediens eggs hatch from April to June. Nymphs hatch and mature in June, emerging as adults shortly after. Winged adults from the progrediens generation then disperse, while wingless ones lay a second generation of 25-125 eggs on hemlock around June (i.e. the sistens generation). Crawlers of the sistens generation begin to feed, but soon enter a dormant stage, called aestivation, lasting about three months. When the temperature cools down in the fall, usually around late September to October, the sistens HWA cease aestivation and begin feeding. They continue to feed and develop through the winter when temperatures are moderate. In the early spring, the sistens adults emerge and go on to lay eggs.
Hemlock woolly adelgid life cycle - timing is approximate and depends on weather & climate.
Because HWA are effectively wingless, they rely on wind, birds, animals and people to move them from place to place. Airborne crawlers rarely disperse more than 600m, though significant wind and rain events are likely to carry them further. Ovisacs are sticky and can easily cling to birds, animals and plants aiding longer distance dispersal. Several bird species, including some ground-dwelling birds, in infested forests were found to be carrying adelgid eggs or crawlers. Ovisacs dislodged from hemlock canopies are found on the trunks of hemlock and other trees within infested forests; eggs and crawlers from the ovisacs were able to survive for up to two weeks in the laboratory. This suggests that early life stages of HWA can be readily spread by animals or by people visiting or working in infested forests.
Temperature is the primary mortality factor limiting HWA populations, this in turn limits its ability to spread to un-infested areas. During the summer, very high temperatures cause HWA mortality however the degree to which this occurs is not yet well-described. Unlike most insects, HWA feeds and develops over the winter, making it vulnerable to cold temperatures; it is thought that cold winter temperatures cause more mortality than hot summer temperatures. Research shows that severity, duration, and timing of cold weather are all important factors. HWA becomes less tolerant of cold snaps as the winter progresses, so colder temperatures later in the winter are likely to have more of an impact on mortality than similar temperatures earlier in the winter. The ability of HWA to tolerate the cold is genetically linked as it varies with geography - adelgids in more northerly sites are better able to tolerate cold. Only a small proportion of HWA need to survive for the population to grow. Studies show that a survival rate of only 2-9% is required to maintain a population, and higher survival rates result in population growth. Colder climates may slow the northward expansion of HWA range, however, population models show that with a warming climate, overwinter survival will increase, and the range of hemlock woolly adelgid will expand northward.
Other factors that affect HWA populations include natural enemies, HWA density and tree health. No predators native to eastern North America exert control on HWA populations, and the adelgid appears to have no parasitoids. Research on classical biocontrol, releasing host-specific predators from Asia or western North America, has been underway for several years but results are not yet consistent. Intraspecific competition does cause HWA mortality, more dense HWA populations are associated with higher HWA mortality.