If a forest is attack by the defoliators how we will measure the population density of the defoliator .
Defoliator populations are detected through aerial surveys and ground reconnaissance. Status of defoliator populations and trends can be determined through predictive and population sampling methods.
Stands appear brownish or scorched immediately after a season of defoliation. Initially, defoliation is concentrated at the tips of branches on current foliage, or at the tops of trees. Primary host trees suffer the most defoliation with other tree species being defoliated to a lesser extent when they are intermingled with the preferred host. Trees will be defoliated in clumps (as occurs with initial stages of Douglas-fir tussock moth outbreaks), or over large areas encompassing one or more stands (as with budworms and loopers). Canopy and understorey trees will have chewed foliage and take on a brownish tinge.
Signs of insect defoliators include:
defoliation
presence of insect life stages
the presence of frass on ground or in foliage
silk webbing, or streamers on ground or in foliage.
It is not possible to calculate the real population densities of defoliators but you can do following things to calculate estimated population densities:
1. collect several "standard" twigs from several trees and count/weight all larvae on those twigs; then you may try to estimate how big is a tree and simply extrapolate the result. One needs to pay attention during sampling on the fact, that larvae usually are not evenly distributed accross the crown of the tree.
2. to simplify the above procedure you may place under forest canopy excrement traps; if the whole sampling design is big enough, then it is possible to find some kind of correlation between the number of insects in tree crowns and weight of the excrements collected underneath; the workload needed for servicing of excrement traps is much lower than sampling of twigs in the bigger area.
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