| The brown planthopper (BPH), Nilaparvata lugens (St l), one of themajor pests of rice in Asia, is a typical r-strategy and monophagous insect species.Its population is affected by climate significantly. In recent years, global warminghas led to frequent droughts and other extreme weather conditions, which are boundto affect the biological characteristics and behavior of BPH, including developmentalduration, wintering boundaries, reproductive rate, host selectivity and feedingbehavior. The climate changes have also affected the population dynamics of BPH.Better understanding of the impact of drought climate on BPH feeding behavior isimportant for its integrated management, and the security of rice production in thefuture. In this study, we used15%(15%P) and20%(20%P) polyethylene glycol(PEG-6000) to simulate different levels of water stress on rice along with noPEG-6000as the control. The study was mainly to evaluate the selectivity of BPH onwater-stressed rice, the differences between the feeding behavior and the honeydewsecretion of the fifth instar nymphs and brachypterou female adults emerged within24hours. Our results are summarized as follows:1. Selection of BPH on water-stressed rice simulated by differentconcentrations of PEG-6000Pairwise comparisons of the insect numbers in each of the three groupsincluding A (15%P/CK), B (CK/20%P)and C (15%P/20%P) showed no significantdifferences (P>0.05) at different times(d) either in fifth instar nymphs or adultfemales within the group; There were no significant differences in the insect numberof fifth instar nymphs between B and C，although the numbers of females showed nosignificant differences between B and C at6h and12h, the number of the insects inCK was significantly higher than that in20%P at24h, and lower than that in15%P.However, the numbers (d=0.05) of the fifth instar nymphs on rice were notsignificantly different between CK and15%P, but the number in15%P was higherthan that in the treatment of20%P (d=1.51). Furthermore, there was no significant difference (d=0.20) in the numbers of females between15%P and20%P, but in thetreatment of15%P the number was significantly higher than that of CK (d=1.87).For both the nymphs and the females, higher numbers of the insect were observed inCK than in20%P. Pairwise comparisons showed that the insect number on theplant was:20%P>CK>15%P.When the results from the three treatments were analyzed all together, resultswere consistent with the results of the pairwise comparisons. However, thedifference in the nymphs between CK and20%P was not significant (P>0.05),whereas the treatments15%P and20%P reached a greater significant level (P<0.01).In contrast, the differences among the female groups did not reach to a significantlevel (P>0.05).2. Comparisons of the electrical penetration graphs of BPH fedwater-stressed rice simulated with different concentrations of PEG-6000Electrical penetration graphs (EPG) showed that in different treatments, thenumbers of BPH, whether the nymphs or the females, sucked in xylem (N5)were higher than those sucked in phloem’s (N4-b), while the feeding time forphloem was much longer than that for xylem. The rank orders of the timelengths in feeding xylem and phloem, and secreting saliva (N4-a) in both thenymphs and females were20%P>15%P>CK, except the nymphs in phloem’sfeeding. Our study also showed that sucking in phloem required salivation to help;however, the xylem feeding generally didn’t need it. Under the water stressconditions, the number of insects requiring the salivation to help their xylem feedingwas increased. Our statistical analyses showed that the xylem feeding time and thenumber of nymphs between15%P and CK, and between15%P and20%P weresignificantly different (P<0.01). However, the difference between CK and15%P infemales was not significant (P>0.05), furthermore, both CK and15%P showedsignificant differences as compared with20%P (P<0.01). The differences insalivation time of both nymphs and females, between the two treatments and CK alsoreached a significant level at P<0.01, whereas the phloem sucking times between theCK and20%P, and between15%P and20%P reached a significant level at P<0.05.Results showed that as compared with CK, the severer the water stress was, thelonger the sucking time needed. Thus, the time for saliva secretion had significantly increased when sucking in phloem (N4-b). In addition, ascompared with the CK, the number of BPH was reduced in different partsaccordingly.The effective sap-sucking rate of N5in both the nymphs and femalesindicated that BPH was likely to stab into the xylem, and females preferred to suck inxylem than the nymphs. Based on our statistical analyses, only the control of thenymphs showed significant difference in comparison with20%P (P<0.01), and therewas no significant difference between the treatments (P>0.05). Based on thetransferring-spine rate of n5, BPH may have a high sucking rate in phloem with nowater stress (CK); however, when there was a water stress, the rate of xylem suckingcan be significantly improved. Nevertheless, the significant difference was onlyfound in the females between the control and20%P (P<0.05).3. Comparisons of salivary flanges number and honeydew secretion of BPH fedthe rice under different PEG-6000concentrationsThe salivary flanges and the total stab numbers in EPG experiments wereconsistent. The salivary flanges of fifth instar nymphs showed the order of20%P>CK>15%P. There were significant differences between CK and20%P, and15%P and20%P (P<0.05). The total stab number reached a significant level(P<0.01), and the numbers of the salivary flange of females were20%P>15%P>CK.The CK showed a significant difference as compared with15%P and20%P (P<0.01).However, the total stab numbers of CK and15%P were not significantly different(P>0.05). There were significant differences among CK,15%P and20%P at P<0.01.Honeydew secretion by nymphs in different treatments showed the order of5d>3d>2d. There was no significant difference between CK and15%P (P>0.05), butthere was a significant difference between CK and20%P (P<0.05). In the femalegroup, except for CK on day3, both CK and15%P showed significant differencesfrom20%P (P<0.05). For the mean honeydew secretion by nymphs and females indifferent treatments, the amount of honeydew was high in15%P, but low in20%P,and there were a little difference between CK and15%P. Particularly in the nymphs,its CK and15%P virtually showed no difference. Based on the salivary flanges andhoneydew secretions, probing and feeding of BPH under CK and15%P water stresswere more obvious than those of the treatment of20%P, which further confirmed the results of EPG.All these results showed that certain levels of water stress (e.g.,15%P) cansignificantly affect the feeding behavior of BPH, through the regulation, however,the insects can tolerant the water stress. Under the severe water stress (e.g.,20%P),the BPH can’t regulate and protect themselves very well. |
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