Characteristics of Eggs and Egg masses of Gastrophysa atrocyanea (Motschulsky) (Coleoptera, Chrysomelidae)

Submitted by helene.luo81 on

本會台灣昆蟲期刊第 43卷第一期第三篇文章由田中誠二博士發表。

以下資料由Dr. Seiji Tanaka田中誠二博士提供:

Adult leaf beetles (Gastrophysa atrocyanea) are commonly observed on dock plants in early spring in Japan. This species has a 1-year life cycle and overwinters as an adult. Adults lay egg masses on the leaves, and the hatched larvae often defoliate dock plants almost completely. Due to this phenomenon, the beetle has been considered a candidate for the biological control of dock plants, which are troublesome in the field.
In this study, I observed the characteristics of eggs, the number of eggs, and the egg masses laid on dock plant leaves to understand the oviposition strategy of this beetle. The color of eggs upon deposition was mostly yellow or golden yellow in most egg masses, but in a few instances, it was greenish, light yellow, whitish, or brownish. However, the significance of this color variation remains unknown. On average, egg masses consisted of 33.8 eggs. The frequency distribution of egg mass sizes exhibited a left-skewed pattern, with a median of 37 and a mode of 44. Surprisingly, there was a positive correlation between egg length and egg mass size.
More egg masses were observed on larger host plants; however, neither the total number of egg masses nor the proportion of leaves with eggs on each plant correlated with the plant size. The results demonstrated that G. atrocyanea females did not exhibit a clear preference for a particular plant size or leaf size as their oviposition site; however, they did show a preference for the underside of leaves.
The distances between adjacent egg masses on a leaf were positively correlated with the leaf lengths and negatively correlated with the number of egg masses per leaf. This suggests that both leaf size and the number of egg masses on a leaf may only establish an upper limit for the distance between egg masses. These findings suggest that the beetles may lay egg masses indiscriminately, regardless of the presence of existing egg masses on the leaves. This behavior may contribute to the occurrence of overcrowding in the new generation, leading to severe defoliation of the host plants.
This study also revealed that G. atrocyanea females did not exhibit a preferred distance between egg masses; however, they rarely laid eggs over previously deposited egg masses on the same leaf. Further investigation is needed to understand the mechanisms by which females avoid depositing their egg masses on top of existing ones on the same leaf. This behavior may be significant in terms of mitigating cannibalism of eggs by larvae emerging from adjacent egg masses.

Seiji Tanaka*。2023。研究蓼藍齒脛金花蟲 Gastrophysa atrocyanea (Motschulsky) (鞘翅目,金花蟲科) 卵和卵塊的一些特性。台灣昆蟲Formosan Entomol.。43: 24-33 (2023) ,DOI: 10.6662/TESFE.202302_43(1).003