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Show 1938] Knowlton, Smith, Harmston: Pea Aphid Investigations 73 When the pea seedlings reached a height of from one to two inches, mature wingless females were placed upon them, reproduction occurring promptly. In this wayan adequate supply of fourth instar pea aphids was always avail able for carrying on the insect predator studies. Syrphidae. During 1937, syrphid fly larvae were the most abundant and effective predators observed in the commercial canning pea and alfalfa fields Table 1 lists the species of adult flies collected during of northern Utah. the season while sampling fields with an insect net to determine the abundance of pea aphids. During favorable periods of the day, in heavily infested pea fields, it was not uncommon to hear a pronounced buzzing produced by dozens of nearby syrphid fly adults which were busily flying about, depositing their It was not unusual to find several eggs among and near to the aphid colonies. syrphid larvae attacking the aphids upon a single heavily infested pea stalk, and to find from one to six or more eggs also present. Syrphid larvae also were noticeably abundant in many pea fields during 1936. Scaeva pyrastri L. was the most abundant syrphid fly adult in pea fields during the early part of the season of 1937, but Eupeodes volucris O. S. took its place as the dominant species during late June and July. Laboratory rearings of syrphid larvae were made, using the method pre viously described. The number of aphids supplied and the number alive at the end of each twenty-four hour period was recorded. Sixty-four adult syrphid fiies were reared from eggs collected among aphids in pea fields, con sisting of 4S Scaeva pyrastri, 16 Eupeodes uolucris, and 3 Syrphus opinator, Table 2 gives the developmental period for two larval Eupeodes volucris under observation for eight days. One larva consumed 20 first instar and 73 fourth instar pea aphids; the other ate 27 first instar and 76 fourth ins tar pea aphids before completing larval growth. Eupeodes volucris in cages completed their life cycle, from egg to adult, in from 18 to 24 days, as follows: eggs hatched in 3 days; larvae pupated in from 8 to 12 days; the pupal period was com pleted in from 7 to 9 days. A syrphid fly larva was observed to crawl about one inch, then attack a winged pea aphid, feeding through an opening it made in the left side of the victim's body, just behind the base of the metathoracic leg. Feeding con tinued for one minute while the abdomen of the aphid gradually shrunk. Then the syrphid predator raised the anterior end of its body into the air and continued to feed upon its prey for another two and one-half minutes. Movements of the aphid appendages gradually stopped as it was worked around, until feeding occurred through a new opening made on the ventral side. The abdomen of the aphid became completely shriveled as feeding at this opening, and later at a point on the thorax, continued for a total of four and one-half minutes. The syrphid larva at times laid out flat, at other times raised its anterior end, together with the aphid, and waved the aphid back and forth as feeding continued. Feeding ended. after a total elapsed time of seven and one-half minutes. The dead aphid still adhered to the predator's body when the latter's mouthparts were removed, so the syrphid larva pushed off the adhering aphid body, then crawled away; waving its anterior end back and forth as it moved along. A third instar pea aphid nymph was taken within a few seconds, the feeding puncture being made upon the left side between the meso- and metathoracic legs. While waving its victim back and forth the predator fed upon the body juices of the aphid nymph for two minutes then discarded the shriveled body. Within a few seconds a third victim was secured; feeding occurred this time upon a winged female, the puncture being made between segments on the side of the ab domen. Wavy and jerky movements occurred during the one and one-half minutes that feeding occurred in this position. The predator reached its : . |
