Additional information about ‘A Successful Biological Enemy for the Peach Stem Aphid Introduced in Yemen’ (Yemen Times, October 6-12, 1997) [Archives:1997/44/Health]

November 3 1997

In the case of the control of the Peach Stem Aphid, it is necessary to explain the concept of biological pest control.
In nature each insect has its own enemies, in the form of other creatures, like birds, spiders and other species of insects that feed on it. Some of these enemies (parasitoids) have evolved so closely with the particular insect that they can only live at the cost of that specific host insect. The result is a natural balance: the insect does not become very abundant and also its enemy does not become too abundant, as otherwise it would wipe out the insect and would have no more food.
However, when the insect is brought, by accident, to another part of the world, it then may multiply very strongly in the absence of its enemy. In that case we speak of an outbreak of the insect. This is what happened with the Peach Stem Aphid in Yemen. The aphid originates from the Eastern part of the Mediterranean region, the Middle East and Central Asia. It must have come to Yemen at least 4 years ago, without its specific natural enemy, a small parasitic wasp (not fly) scientifically called Pauesia antennata. This wasp lays its eggs in the bodies of the aphids and the wasp’s offspring devour the aphids from within. Later the aphid dies and a new wasp emerges from its body.
The absence of this natural enemy in Yemen led to the enormous outbreak of the Peach Stem Aphid. At first, it was tried to control the pest by chemical pesticides. This is a very costly practice and has to be repeated regularly to keep the pest under control. In addition, the frequent application of chemical pesticides has negative side effects on the environment and in some cases may compromise human health. Later, with the support of the Dutch Government, Food and Agriculture Organization of the United Nations, German Agency for Technical Cooperation and International Institute of Biological Control, the Plant Protection Department of the Ministry of Agriculture and Irrigation managed to obtain a number of the above-mentioned wasps from Pakistan in order to multiply them and release them in the affected areas.
Since January a tremendous effort made at the facilities of the Plant Protection Department at Shoub resulted in the production of over 150,000 wasps, that were released first in the Sana’a surroundings and later in localities as far apart as Amran, Manakha, Ibb, Mukeiras and Dhamar. The wasps have been very successful in controlling the pest and although the multiplication and release of the wasps continues, the problem has much diminished and millions of almond, peach and other deciduous trees were saved from certain death. The final result will not be the total eradication of the Peach Stem Aphid. It is expected, that by next year a natural balance will be reached between aphids and wasps, the aphids remaining present in the orchards but never reaching the enormous quantities that led to the outbreak of the last years. Before pest numbers will rise so high as to cause damage, the wasps will arrive and destroy part of the aphids.
The sole side-effect of the introduction of a very specific biocontrol agent like Pauesia, will be that one more insect species is added to the Yemeni fauna, to join the tens of thousands of other insect species already present. Now that the Peach Stem Aphid in Yemen is returning to normal (low) proportions, the number of Pauesia wasps will also become much lower, as the wasp can only multiply by laying its eggs into the aphids. It cannot lay its eggs in other insects or animals and it cannot eat other food than just Peach Stem Aphid. Therefore, concern about what will happen now, is unfounded.