Studies On Population Dynamics Of Aphids And Interactions With Their Natural Enemies In Cornfield

Corn aphids are the most important pests of corn in China; they have been outbroken in major corn planting regions in recent years and caused heavily yield losses. In order to effectively prevent the damage of aphids on corn, it is essential to learn the population daynamics of corn aphids and the natural enemies in cornfield as well as their interacions. This paper investigated the species, the population dynamics and the natural enemies of corn aphids in cornfield in Langfang, Hebei province, and the predation of dominant predator to Rhopalosiphum maidis- Propylaea japonica has also been studied. The main results are as follows:1. There are 5 species of aphids occurred in the cornfield in Langfang, Hebei Province, Rhopalosiphum maidis (Fitch), R. padi (L),Macrosiphum miscanthi (Takahashi),Schizaphis graminum (Rondani) and Aphis gossypii Glover, respectively, by field survey from 2009 to 2010. Temporal distribution: Corn aphids in spring cornfield have two peaks, Macrosiphum miscanthi in early June, and Rhopalosiphum padi and Aphis gossypii together in mid-August, respectively. Although Macrosiphum miscanthi is the dominant species in spring cornfield, but it mainly occurs in corn seedling stage, and has little effect on corn yield, so we should pay particular attention to aphid Rhopalosiphum infestation. Aphis gossypii is the dominanat species in early development stage of summer corn, then follwes by mixed populations of Rhopalosiphum maidis and R. padi in the middle and late development stage, they are the main populations of corn yield.Spacial distribution: Macrosiphum miscanthi mainly feeds on the lower leaves of corn plants, Rhopalosiphum maidis mainly feeds on the top part in tasselling stage and then moves to middle part of corn in late season. R. padi feeds on the middle part of corn in late developmental stage of the plants. Aphis gossypii mainly feeds on the lower leaves in the early stage of corn, and gradually moves upward to the middle and top leaves as the lower leaves dry and fall off in late developmental stage of plants.2. Field survey from 2009 to 2010 showed that there are rich of natural enemies of corn aphids in cornfield in Langfang, Hebei province, totally including 6 orders, 15 families, and 25 species of natural enemy insects, at the same time there are many predatory mites. Among all of them, ladybug, lacewing and flower bug are the dominant species. Ladybugs mainly include Propylaea japonica (Thunberg) and Harmonia axyridis (Pallas). The number of Propylaea japonica is much larger than that of Harmonia axyrid in spring cornfield, while in summer cornfield, Propylaea japonica and Harmonia axyridis dominates in the early and late developmental stage of corn, respectively. Chrysoperla nipponensis (Okamoto) is the most abundant species of lacewing.The spacial distribution: Propylaea japonica occupy the upper part of corn in spring cornfield and moves downward to middle and lower part of the plants in summer cornfield; Harmonia axyridis is on the middle part of corn in the late stage of summer corn; the lacewing is evenly distributed all over the corn plants; and flower bug is mainly on the upper and middle part of corn. 3. The correlation analysis on the population of corn aphids and their natural enemies indicated that the natural enemies of aphids play a predominant role in controlling aphid occurrence. As the dominate Macrosiphum miscanthi occurs earlier in the spring cornfield, the control effection of natural enemies on aphids shows negative correlation (r =-0.73839, p=0.0003): the predators, such as ladybug (r=-0.60876, p=0.0057), lacewing(r=-0.71354, p=0.0006), and flower bug (r=-0.70799, p=0.0007), they play an obvious role in controlling the corn aphids. Other natural enemies, such as spiders (r=-0.36793, p=0.1212) are relatively minor in controlling the aphids. Parasitic wasps (r=0.61042, p=0.0071) showed a better effect in controlling Macrosiphum miscanthi.In the late reproductive stage of summer corn in 2009, the population of the natural enemies rapidly increased with the rise of aphid population, and then the aphids were effectively controlled by the natural enemies (r=0.59039, p=0.0126). The occurrence peak of ladybug, flower bug, and parasitic wasp population are almost identical with that of aphids, they both peaks on September 13th. Ladybug showed the best control effect on aphids (r=0.70340, p=0.0016), followed by spider (r=0.60064, p=0.0108), flower bug (r=0.51399, p=0.0348), parasitic wasp (r=0.46653, p=0.0591) and lacewing (r=0.34787, p=0.1712).As a large number of the natural enemies of aphid exist before the aphid population inceased, so the aphids kept under control in early developmental stage of summer corn in 2010. Later, as the number of aphids increased, the parasitic wasps increases and inhibited the aphids effectively (r=0.81146, p= 0.0001). Parasitic wasps (r=0.58136, p=0.0182) showed the best effect in controlling aphids; followed by ladybug (r=0.22756, p=0.3967), lacewing (r=0.34331, p=0.1930), flower bug (r=-0.30316, p =0.2537) and spider (r=0.48767, p=0.0553). It indicates to some extent that the low aphid density in the summer corn field in 2010 is the result of the combined function of all natural enemies (r=0.81146, p=0.0001).4. The predatory function of Propylaea japonica adults and larvae to corn leaf aphid, Rhopalosiphum maidis, were tested in laboratory. Results showed that there were significant differences between different instars of larvaes and adults of P. japonica in their daily preying capacity for R. maidis. The preying capacity of 3rd and 4th instar larvae of Propylaea japonica is much better than that of 1st and 2nd instar larvae, and the female adult's preying capacity is better than that of male. The 4th instar larvae of P. japonica had a strong predatory function to R. maidis and the theoretical daily maximum prye number of aphid was 196.85. Following female >male>3rd instar>2nd instar>1^st instar. The functional responses of the adults and larvae of P. japonica to the density of R. maidis were in accord with Hollingâ…¡.χ² test showed that the theoretical expectation was identical to the observed values. The preying action of Propylaea japonica in a certain space was limited by both preydator and host density. Mutual interference among individual predators and preys could be described by Hassell equation.