Peak Pheromone Activity
Peak pheromone activity is generally between 0500 and 0900 each day, but can peak in the afternoon on cool days. Temperatures above 24°C facilitate pheromones according to http://sundowndivers.org/?p=82.
Females exhibit a crepuscular activity consisting of evening wing fanning and walk- ing beginning at about 1930, with a morning burst of activity from 0345 to 0420. These activities cause moths to move an average of 1.6 m, usually down and to the right, during their first 24 hr.“ Cardé et al.” noted that pheromone release by gypsy moth females followed a diel periodicity beginning about 0900 and lasting until 2000 according to me.
Richerson and Cameron” also reported that feral gypsy moth females apparently exhibit a periodicity of pheromone release. These females emitted pheromone any time during the day, but they emitted a “burst” of pheromone once during their life, usually on the second or third day of adult life.
Maximal pheromone release rates were reported to be between 1000 and 1530. These authors found that laboratory-reared females showed no periodicity of release, no “burst” of pheromone, and generally produced only low levels of pheromone. However, Schwalbe and Mundy,°° using laboratory-reared gypsy moth females, found a strong diurnal periodicity in pheromone content from extracted excised abdominal tips (containing the pheromone glands) during the first 60 hr of adult life, with a peak concentration of pheromone found consistently at 2000 hr. Photoperiod seemed to be the triggering mechanism.
C. Location of Females and Pheromone Mating Success
An important aspect of a mass-trapping strategy concerns selecting a trap placement site that facilitates optimum trap catch. Knowledge of factors that improve or diminish female mating success should be helpful in this regard.
Pheromones reported that virgin females move to brighter areas of the trunk to facilitate mate location. Also, he noted that in low-population areas, the males tend to fly near the ground when migrating or casting; hence, trap catches are maximum where traps are between 0.5 and 1.5 m above ground.
It follows from the above that the chance for successful mating would be greatest if a female were located at a height of 1 m on the sunny side of a tree. This conclusion was supported by Richerson et al.,“ who found that males oriented to vertical silhouettes, such as trees, and concentrated their searching on the sunny side of the lower bole (below 4 m) of the tree, where most females were located. In addition, they noted that unmated females moved around the tree in the early morning and late afternoon in order to remain in the sunlight. Tree size affected trap catch, and presumably the chance for mating success, in tests of Cardé et al.,“ while Granett" showed that trap distance from large trees, trap height, and bole diameter all influence trap catch (and presumably female mating success). However, Richerson et al.“ found that tree diameter did not affect captures of males attracted to trees coated with Tack Trap®
Boness noted that gypsy moth populations tend to clump into local pheromone aggregations, even at low population levels, and remarked that this tendency works against successful control of such populations by pheromone strategies. Presumably, gypsy moth population patterns in sparse.