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

## TL;DR

This study examines the population patterns and aggregation behavior of a pest, Galerucella birmanica, in Brasenia schreberi aquaculture, identifying a chemical attractant from damaged leaves.

## Contribution

The study identifies 2-phenylethyl isothiocyanate as a key attractant for G. birmanica, offering a new approach for pest management in B. schreberi cultivation.

## Key findings

- G. birmanica showed a 21.6-fold higher abundance in severely chewed leaf areas compared to non-chewed areas.
- 2-phenylethyl isothiocyanate significantly attracted G. birmanica adults in laboratory tests.
- Damaged B. schreberi leaves release herbivore-induced volatiles that may protect the plant population from pests.

## Abstract

The aquatic macrophyte watershield, Brasenia schreberi Gmel., with its young buds coated in a thick mucilage served as a famous vegetable, has been cultivated in China for a long time. However, its production has been threatened by a pest, Galerucella birmanica Jacoby. This study investigated the population dynamics of G. birmanica throughout the entire growth season of B. schreberi from May to November and identified the aggregation behavior of the pest, with a preference for severely chewed leaf areas. Further analysis revealed that it was 2-phenylethyl isothiocyanate released from the damaged leaves that attracted G. birmanica. Our findings provide valuable insights for pest management in B. schreberi cultivation fields, and the attractant effect of 2-phenylethyl isothiocyanate on G. birmanica offers a new perspective for the development of attractants for this insect.

The aquatic vegetable Brasenia schreberi Gmel. (Nymphaeales: Cabombaceae), widely cultivated in China, faces severe challenges from pest disturbances. With a field investigation, G. birmanica occurred for several generations in the Brasenia 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. G. birmanica exhibited a conspicuous aggregation in distribution, with severely chewed areas having a G. birmanica 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 G. birmanica adults. Our study suggests that G. birmanica can be a severe threat to mono-cultivated B. schreberi, 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.