Responses of Phlox subulata aboveground to root-feeding Scarabaeidae larvae and drought stress

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The ornamental perennial Phlox subulata and its varieties are often affected by root-feeding Scarabaeidae larvae, leading to dieback. These larvae are harmful insects that are difficult to detect. The relationship between the horizontal distribution of root-feeding Scarabaeidae larvae and the aboveground condition of P. subulata was examined in the field. Laboratory experiments were conducted to examine the effects of drought stress, root-cutting, and larvae introduction on the leaf chlorophyll index (LCI) in P. subulata. A field survey was also conducted, and the numbers of larvae were found to be higher in plots where P. subulata experienced dieback than in plots where the plants did not die, and in plots adjacent to dieback plots. This result indicates that the number of larvae corresponds to the aboveground condition of the plant. Both root-cutting and drought stress treatments significantly decreased the LCI and killed some of the plants, implying that drought stress and physical root injury changed the plants’ aboveground condition. The introduction of larvae also significantly decreased the LCI, in a manner similar to the drought stress treatment. The decreases in the LCI due to the drought stress and larvae introduction treatments were detected before the dieback of P. subulata in the experiment, which implies that constant monitoring of the LCI is an effective method for early detection of root-feeding larvae.