Document Type : Research Article
Department of Plant Protection, Faculty of Agriculture, Bu-Ali Sina Univ. Hamedan
The competition for limited resources is a common ecological interaction among animals. In most of insect parasitoid communities, different species compete for specific resources both in larval and adult stages. Intraspecific competition play a role in the size, structure, stability of insect communities and even it determines the fitness of species. Moreover, understanding how competition influences on different insect species is essential for basic ecological studies and pest control issues. The outcome of competition between adult parasitoid insects depends on host finding, ability to disperse, reproductive capacity, ability to fight, and physiological coordination with the host.While the outcome of competition in the larval stage can be influenced by differences in the growth rate of the parasitoid, the stage and physiological state of the attacked host, the order and the time interval between oviposition and the evolutionary history of the species. Competition between larvae can affect the development of adult parasitoids because the surviving individual or winners might to pay high costs for competency due to quantitative and qualitative changes in host resources.
Materials and Methods
The current study was carried on to determine the effect of intraspecific competition on searching efficiency and oviposition strategy of Habrobracon hebetor Say on 4th instar larvae of Heliothis viriplaca (Hufnagel, 1766) (Lep.: Noctuidae) at seven competition and four adult density levels (1, 2, 3 and 4) of parasitoid wasps per the spring chickpea plant (c.v. Bionij). All plants were grown under controlled climate conditions (25 ± 1 ° C, and a light period of 16: 8 hours). Then, 10 fourth instar larvae of H. viriplaca per plant were released and allowed to feed and establish prior starting the experiments (about three hours). Then, fertile female wasps with different densities (2, 3 and 4) in separate treatments and a control treatment without the presence of a competitor (one fertile female wasp per plant) introduced to each microcosm unit and after 24 and 48 hours, the number of parasitized and oviposited larvae on each host plant counted, but the number of eggs oviposited on each host larvae recorded at the end of 48 hours. All eggs laid at each competitive level were kept separately until the emergence of adults to record sex ratio and mortality rate.
Results and Discussion
The results showed that, the searching efficiency of H. hebetor decreased with increasing the level of competition, so that, the highest searching efficiency was recorded after 48 hours, in non-competitive treatment (control) and at a density of four parasitoid individuals as 0.178 ± 0.002 /hour and the lowest value at the level of competition 12 and density of two parasitoid females as 0.023 ± 0.004/ hour. As the density levels of parasitoid increased, the searching efficiency was decreased to a density level of three, but then it increased at four individuals per chickpea plant. The interaction effect of density × competition was not significant, in other words, these two factors independently influenced on searching efficiency of H. hebetor. Time had a positive effect on searching efficiency so that, at all levels of competition and at different densities of parasitoid wasp, the difference was statistically significant. With increasing density of H. hebetor per plant, at all competition levels, the oviposition rate was increased, which means that the simultaneous presence of several parasitoid individuals does not have any negative effect on oviposition rate. Competition had a negative effect on the oviposition rate of female H. hebetor, as the highest oviposition rate recorded at one individual and lowest competition level as 14.7 ± 2.1 eggs and the lowest rate occurred at four individuals per host plant and competition level of 12 as 6.4 ± 0.05 eggs. According to linear regression line equation, the interference coefficient was calculated as -0.134. The negative slope of the regression line indicates the existence of mutual interference between the H. hebetor adult females. In other words, the interaction had a slight negative effect on parasitoid searching. At all competition levels, regardless of density, sex ratio was not affected by competition and other factors sounds influential in this regard, however, the difference between the ratio of female to male population at competition levels of one and two was greater than other levels. Moreover, the mortality rate has increased with the increase of the level of competition, so that the lowest mortality percentage occurred at the level of one and two and the highest at the level of competition 10 and 12.
The occurrence of interference between parasitoid wasps in laboratory environments could not be used as indicative of real conditions, because high pest densities in field conditions moderate the intraspecific competition intensity among parasitoid individuals. Given the significant differences in searching efficiency and other factors studied, it can be concluded that the number of competing larvae in a host can affect the suitability and vigor of adult parasitoid wasps. It should be noted, however, that in some cases competition does not directly affect searching capability, but instead, by reducing other factors, such as morphological characteristics (body size, ovary size, number and size of eggs, size of hind legs, wing size), number of embryonic eggs, oviposition days duration, and other biological characteristics will influence on the searching abilities of parasitoids indirectly.The study of H. hebetor parasitoid wasp foraging behavior showed that in high densities, it avoids competing with other conspecific individuals. Anyway, regarding the prevalency of competition among released natural enemies in greenhouses and fields, the study of this interaction from laboratory to fields is being recommended.