Timing is everything: Improving predictions of winter New Zealand grass grub densities and associated damage from summer and autumn larval counts

Costelytra giveni is a serious pasture pest in New Zealand and accurate estimates of population densities are important to inform control measures. This species generally has a one-year life cycle so populations should either remain stable after eggs have hatched or decline due to larval mortality. Larval counts were obtained using a simple, standard and widely used sampling method from a series of soil cores collected from ryegrass research plots in Canterbury, New Zealand between 27 January and 16 June 2021 and a significant increase in population was recorded. Measurements on 27 January, 19 March and 5 May, represented only c. 8%, 25% and 63% of the mean densities measured on 16 June, respectively. The apparent increase in larvae is attributed to failure to find small 1st and 2nd instar individuals within the soil samples. Larvae increased in size as they transitioned from 1st to 3rd instar and later instar specimens were more easily discovered. An equation to describe the observed results provided date-related correction factors to allow a more realistic prediction of C. giveni larval densities in the winter following empirical larval counts. Larval counts measured on 27 January, 19 March, and 5 May, would need to be multiplied by 13, 4 and 1.6, respectively, to accurately estimate the larval density found on 16 June. This study showed that summer-autumn sampling using the current method can significantly underestimate winter C. giveni larval densities, potentially leading to unanticipated pasture production losses. Similar results were also found in 2022. While the equation provides a guide to population estimates, the caveat is that region and environment will influence population trends in any particular year.