Bionomics of the whitefly parasite complex associated with cotton in southern California (Homoptera: Aleurodidae; Hymenoptera: Aphelinidae)

Three species of whiteflies, Aleyrodes spiraeoides Quaintance, Bemisia tabaci (Gennadius), and Trialeurodes abutilonca (Haldeman), infest cotton in the Coachella and Imperial Valleys of southern California. From February to April no green cotton leaves are available. A. spiraeoides is found during that period on Sonchus oleracens L. , B. tabaci is found on Malva parviflora L. , and T. abutilonea is found on a variety of plants, including Pluchia scricca Coville. Six species of parasites are known to attack the whiteflies. Encarsia formosa Gahan, E. mcritoria Gahan, and Eretmoccnts haldemani Howard parasitize all 3 whitefly species; Encarsia coquillctti Howard parasitizes A. spiraeoides and T. abntilonca; and Encarsia sp. and the females of Prospaltclla species z. attack only T. abntilonca. Females of all parasite species and males of E. haldemani are solitary, internal parasites of whitefly nymphs; males of Encarsia spp. are parasites of Encarsia females; and males of Prospaltclla z. are parasites of eggs of Lepidoptera. Of the whiteflies, A. spiraeoides is rare, B. tabaci predominates the whitefly fauna in the Coachella Valley from July on, and T. abjitilonca predominates the whitefly fauna in the Coachella Valley until July and is predominant throughout the season in the Imperial Valley. Encarsia sp. and E. coquillctti are rare; E. formosa and Prospaltclla z. occur in low numbers. E. mcritoria occurs in low numbers in the Imperial Valley, but is abundant during the first two-thirds of the season in the Coachella Valley. Eretmoccnis haldemani is the major constituent of the parasite fauna in the Coachella Valley during the late summer, and in the Imperial Valley throughout the season. Treatments with both trichlorfon and a combination of methyl parathion and endrin greatly reduced the adult populations of whiteflies and parasites. The reduction caused by trichlorfon was usually less drastic than that by methyl parathion -fendrin. The whiteflies and parasites usually regained their pretrcatment population levels 2 weeks after treatment. Cotton is grown in southern California primarily in the Coachella and Imperial Valleys. Three species of whiteflies, Bemisia tabaci (Gennadius), Trialeurodes abutilonea (Haldeman), and Aleyrodes spiraeoides Quaintance, infest cotton in these areas. These insects do not usually cause economic damage. When such damage occurs it often follows an insecticide application on or near the field in question, and is most pronounced near the margins of the field or in unusually poor cotton stands. These facts suggest that natural control factors play an important role in keeping the whitefly populations at low levels. This paper reports the results of biological and ecological studies of the aphelinid parasites that attack the aforementioned whiteflies. MATERIALS AND METHODS All plant rearing and laboratory studies with hosts and parasites were conducted in the greenhouse Capprox temp range winter, 74°-80°F; summer, 60°90°F; and relative humidity range, 62%-66%) unless specified otherwise. The cotton variety Delta Pine Smooth Leaf was used as a host plant; A. spiraeoides, the iris whitefly; B. tabaci, the sweet-potato whitefly; and T. abutilonea, the banded-wing whitefly, were used as host insects. The host and parasite material were collected in the Coachella and Imperial Valleys, either as free-living adults or as immatures on the cotton (eaves. In addition, T. vaporariorum (Westwood), the greenhouse whitefly, obtained from local infestations, was used for some host-parasite specificity studies. Host and parasite populations were sampled over a 1 Accepted for publication September 30, 1966. 3 Formerly Post-Graduate Research Appointee, Dry Lands Research Institute, University of California, Riverside. Present address: Department of Zoology, Tel-Aviv University, Tel-Aviv, Israel. period of several years and under a variety of conditions : 1. The Coachella Valley. —All samples were taken in Indio from both untreated fields (1962 through 1965) and treated fields (1963 and 1964). All treatments were made with trichlorfon, which was purchased under the trade name Dylox®. 2. The Imperial Valley. —Sampling was clone in Westmoreland in an untreated field, a field treated with methyl parathion + endrin, a field treated with trichlorfon, except for the last 2 treatments which were made with methyl parathion + endrin (1963), and a field treated with trichlorfon only (1964). Numbers of both whiteflies and parasites were determined from weekly suction samples collected with a "D-Vac" suction machine. The weekly samples were taken during the morning hours and from the same general area within the field, and consisted of 100 suctions each, i.e., each was a pooled sample from 100 individual plants. This sampling was always followed by separation of the insects from the debris accumulated during the sampling process. A Berlese-type funnel was used for separation ; the insects were then placed and kept in alcohol. Percentage parasitism was calculated from the sum of all whiteflies and parasites found in the sample. Because each whitefly pupa gave rise either to 1 whitefly adult or 1 parasite, this sum represented the total number of whitefly pupae from which the sampled population emerged, provided the samples were taken from a field that was not treated with insecticides.