Effect Of Multiple Hosts Environment On Host Selection Behavior And Fecundity Of Plutella Xylostella (Lepidoptera:Plutellidae)

Insect herbivores often have higher densities on host plants grown in monocultures than those in diverse environments. The effect of host plants on the fertility of female adults of the diamondback moth, Plutella xylostella (L.) was studied. To elucidate the underlying mechanism, the development of ovaries, quantity of the energetic substances and mRNA transcrition of genes such as vitellogenin and NADH in fatbodies of female P. xylostella adult reared in different host plants were compared. Based on these, the possibilities of controlling P. xylostella through mix cropping host plants were tested in the field.1. In monoculture, female P. xylostella adults preferred Brassica campestris L to B.juncea Coss and B. oleracea L. In multiculture, the take-off of female P. xylostella adults in48h is3.1times more frequent than that from the B. campestris monoculture environment and2.9times of the B.juncea monoculture environment. On the contrary, with the lowest takeoff frequency in B. campestris,57.4%more eggs were laid compared with that in multiculture. Similarly,19.0%more eggs were laid in B.juncea while the number of eggs laid in B. oleracea was near to that in multiculture. All these indicated that increase in take-off would decrease the fertility of female P. xylostella adults.2. The behavior assays showed that in case of a single host plant species,72.7%of the female P. xylostella adults visited the host plants quickly due to the indicator of volatile chemicals released from host plant. But when more host plant species were presented concurrently, no significant differences were determined among the numbers of P. xylostella individuals on each species of host plants. This implied that the volatile substances were not the sole factor mediating P. xylostella host plant location. In contrast, after landing on the surface of the host plants, the tarsal sensilla played a greater role than the ovipositor sensilla in host plant selection.3. The multiculture of B. campestris with B. juncea could significantly decrease the number of eggs by41.6%compared with that of B. campestris monoculture and30%compared with B. juncea monoculture. However, after mixing B. campestris with B.juncea, the number of eggs changed little, but the distribution of eggs varied greatly for84%of the eggs were found in B. campestris which resulted in the protection of the B. juncea. So, when the difference of P. xylostella’s preference to the host plants was small, host plant mixing would disrupt the egg laying of P. xylostella. Meanwhile, when that was large, then that would become trap effect.4. The field experiments showed that compared with monoculture, multiculture could inhibit the P. xylostella population. Based on the life table results, the population trend index (1) of P. xylostella in multiculture fields was significantly lower than that in monoculture fields. The multiculture of B. campestris and B. juncea at the ratio of1:1brought about the most significant inhibition effect. The plant distance no significant effect on the P. xylostella number if host plants were within2m from each other.5. The longest ovaries (3.401±0.119mm) were found in the female P. xylostella adults from B. campestris monoculture and the shortest (2.180±0.088mm) were in those from multiculture, which were35.9%shorter than the longest ones. The interrelationship between the take-off frequency and ovary length could be summarized as y=-0.0055x+6.2122(R2=0.9671), suggesting that increase in take-off frequencies would shorten the length of the ovaries of female P. xylostella adults. The glyceride contents in the female P. xylostella adults from B. campestris and B. juncea were the same, but over2times than that in those from B. oleracea. Meanwhile, glyceride content in the female P. xylostella adults from multiculture plant hosts was comparatively low, about the content in those from B. oleracea. RT-qPCR analysis revealed that the expression level of NADH (px016952) was positively related with take-off frequencies and a negative relationship was determined between the expression level of NADH and vitellogenin gene (y=-0.7812x+1.6202,R2=0.5811)In summary, multiculture of similar host plants significantly affected the amount of eggs laid by female P. xylostella adults. The mechanisms underlying maybe is the increased in take-off frequency and consumed more energy substances (glyceride), the much of glyceride be consumed resulted Vg synthesis decreasing and ovarian development be influenced, at end the insect appeared the fecundity decreased. So the present studies further explain the underlying mechasims of the effect of host plants on egg laying of P. xylostella., whichprovide both the theoretical and practical evidence for decreasing P. xylostella population by means of multiculture.