Graduate Thesis Or Dissertation
 

Interspecific competition among four species of grain beetles at three temperature and moisture levels

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  • This study was conducted to test the performance of four species of stored grain beetles in competition at three levels of moisture and temperature. The insects used in these tests were the granary weevil, the rice weevil, the saw-toothed grain beetle and the red flour beetle. The experiments were conducted at temperatures of 90, 80, and 70 degrees Fahrenheit and at moisture levels of 15, 12, and 9 percent moisture content in two varieties of commercial wheat. Redmond variety commercial feed wheat was used for the majority of these tests. Several of the tests were duplicated using Idaed variety seed wheat. The competition studies were designed to test the performance of one species of grain beetle when reared in the presence of one or two other species. The procedure used was to obtain 50 adult insects of each species from stock cultures. These insects were picked at random and introduced into 250 gram samples of wheat in one pint containers and allowed to remain for one week. The initial adult population was then removed and the adult progeny were counted periodically to determine the size and time of appearance of the first generation. The following combinations of insects were tested: 1. Granary weevil and saw-toothed grain beetle 2. Granary weevil and red flour beetle 3. Saw-toothed grain beetle and red flour beetle 4. Granary weevil, saw-toothed grain beetle and red flour beetle 5. Rice weevil and saw-toothed grain beetle 6. Rice weevil and red flour beetle In addition to these combinations, all species of insects were reared alone in each test in order to compare the performance of the insects when in pure culture and when in competitive situations with the other species. The rice weevil was the best competitor of the four species and was less influenced by the presence of other species of beetles. The rice weevil has a wide range of temperature and moisture tolerance and, though considered a tropical species, it is a common and successful pest of stored grain in the Pacific Northwest. The granary weevil was a good competitor because of its capacity to attack whole grains but did riot do as well as the rice weevil, probably because of its lower reproductive rate and longer life cycle. The granary weevil and the rice weevil were well adjusted to the conditions of these tests because of their ability as primary feeders in whole grain. Competition between weevils and bran bugs (the sawtoothed grain beetle and the red flour beetle) usually favored the bran bugs since the weevils chewed up the grain and made food material available to the bran bugs. In competition with weevils, the bran bugs usually appeared earlier than the former but ordinarily had smaller populations. In competition between bran bugs, the red flour beetle was almost always the dominant species. This was because of the ability of the red flour beetle to attack whole wheat grains and because of its extensive predation of the eggs and pupae of the saw-toothed grain beetle. The saw-toothed grain beetle and the red flour beetle usually produced larger populations in the presence of the rice weevil than when with the granary weevil. Where conditions of high temperature and moisture were present in the grain, mold often formed. The presence of the mold may have been favorable to the two species of bran bugs. It has been observed that when the granary weevil or the rice weevil is reared alone under high moisture conditions, there is considerable mold growth. When, however, the weevils are reared in the presence of one or two species of bran bugs, little or no mold is found. It is not known if the bran buns actually feed ori the mold but the results of these tests indicate that both species of bran bugs either feed on the mold or in some way inhibit its formation in grain at high moisture and. temperature levels. This work was performed as part of regional project WM-16 (maintaining marketability of stored wheat through the control of insects) and was conducted under an assistantship granted by Oregon State College, Department of Entomology, Corvallis, Oregon.
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