The tiny white insects that swarmed the capital Tehran in the past summers have not been present so far this year, much to the relief of the residents who already suffer such distressing metropolitan challenges as horrendous traffic jams and air pollution.
Although no definite reason for their presence, and now absence, is known, experts and officials have presented a number of theories.
Alireza Davoudnejad, an environment expert at Tehran's office of the Department of Environment, has highlighted three factors which are chiefly responsible for their invasion, namely changes in the ecosystem, expansion of their host plant and warm weather.
"Excessive spraying of pesticides had eliminated the species that feed on whiteflies, thus allowing them to increase in number," he said, Zist Online reported.
Besides, paper mulberry, an endemic East Asian tree that plays host to the insect, has been planted widely across Tehran in recent years offering abundant lodging for whiteflies, he added.
Unprecedented high temperatures plus badly collected urban rubbish, according to him, made the conditions even more favorable for the insect's survival.
Shahrokh Pashaei Raad, a biology and entomology professor at Shahid Beheshti University, had earlier attributed their absence to the delay in the arrival of warm weather since whitefly eggs hatch with the rise in temperature.
"Lack of springtime rain and early summertime heat had accelerated their spread in the past years," he said.
He had, however, noted that they were lurking and could appear any time because their host is still out there.
Esmaeil Kahrom, an expert and adviser with DOE, postulates that the dry days in Tehran could possibly have caused their disappearance.
"Flies and mosquitoes strongly depend on water because they lay their eggs in water," he explained.
He pointed out that the variety of wildlife naturally changes in cities and this is true for insects too especially because they are light and easily carried away by wind.
Control Measures
The head of the health and environment committee of Tehran's City Council, Zahra Sadre-Azam Nouri, put it down to a set of eight urban management measures to control the population of whiteflies.
"Since the early days of the new council's term, we put the issue on agenda and hired professional advisers who eventually came up with eight guidelines including physical and chemical control, wash-up and alteration of plant species," she said.
The most effective move, she said, was gaining entry into certain private areas.
"In the past, entry restrictions disrupted the procedures and did not allow the plans to be completely implemented," she said.
Nevertheless, Nouri noted that whiteflies have not entirely vanished but their explosive growth has been curbed.
Whiteflies are small insects that go into a state of hibernation in winter and rise again in the summer. Although they can weaken and finally kill plants, they have been proven harmless to humans.
Pruning infected trees and using sticky traps or natural predators as well as herbal and chemical pesticides are among the methods to control the insect's growth.
