Population Dynamics of Galerucella birmanica and Its Aggregation Behavior in Brasenia schreberi Aquaculture System

The aquatic vegetable <i>Brasenia schreberi</i> Gmel. (Nymphaeales: Cabombaceae), widely cultivated in China, faces severe challenges from pest disturbances. With a field investigation, <i>G. birmanica</i> occurred for several generations in the <i>Brasenia</i> mono-cultivation system. The first visual peak on leaf surfaces appeared in July, with eggs, larvae, pupae and adults all being recorded. The highest number of flying adults were trapped in August in reverse to the absence of beetles on leaves, followed by an extraordinary high number of eggs in September. <i>G. birmanica</i> exhibited a conspicuous aggregation in distribution, with severely chewed areas having a <i>G. birmanica</i> abundance 21.6 times that of the non-chewed areas. Laboratory studies with GC-MS and two-choice tests revealed four volatiles significantly discrepant in contents released from chewed/intact leaves: cis-3-hexenyl acetate (917.33 ± 29.56 vs. 604.034 ± 23.24 ng, chewed vs. intact), 2-phenylethyl isothiocyanate (595.37 ± 28.42 vs. 356.00 ± 13.44 ng), undecane (771.44 ± 34.72 vs. 1003.28 ± 47.88 ng) and methyl salicylate (1079.84 ± 49.39 vs. 532.11 ± 18.23 ng); among them, 2-phenylethyl isothiocyanate could significantly attract <i>G. birmanica</i> adults. Our study suggests that <i>G. birmanica</i> can be a severe threat to mono-cultivated <i>B. schreberi</i>, whereas the damaged leaves may sacrifice themselves by attracting the beetles with an herbivore-induced volatile, thus protecting the whole plant population from pest disturbance.