The Population Dynamics of the Blue Willow Beetle on Short Rotation Coppice Willow in the Scottish borders.

Abstract

The life cycle and life table of Phratora (=Phyllodecta) vulgatissima (L.) was investigated from 1996-2000 at two short-rotation coppice willow (Salix viminalis clones) plantations in the Scottish-English borders. The first site 'Turvelaws' is a heavily infested site, while 'Rutherford' has low infestations of P. vulgatissima. Overwintered adults start emerging in mid March and continue to migrate into the plantation until mid May, with the peak emergence typically occurring in mid April. Spring canopy densities of adults reached levels of 28.4 m^-2 in 1997, 44.3 m^-2 in 1998 and 7.93 m^-2 in 1999 at Turvelaws. After feeding for a couple of weeks the adults copulate; the first eggs are laid in early May. The egg laying reaches a peak in mid May, but eggs are present in the plantation until late July. Larval stages are typically found from late May-July, unless there is a second generation, in which case larvae can still be found up until September. Numbers of first instar larvae peak early in June, second instars peak in mid June and third instars in early July. Numbers of eggs and larvae peaked again during August 1997, suggesting a partial second generation. It was previously thought that P. vulgatissima unlike P. vitellinae had only one generation per year (Kendall and Wiltshire, 1998). Pupae can be found from the start of July until mid September, their numbers peaking in mid July. The imagines start to emerge in late July, and their numbers peak in mid August, with densities of over 100 m^-2 recorded at one site. The imagines feed for a couple of weeks and then leave the plantation to find suitable overwintering sites. From the key factor analysis of 3 generations of P. vulgatissima at Turvelaws, the key factor was identified as adult overwintering loss. This is the disappearance of adults between leaving the plantation in the autumn and returning in the spring. A correlation coefficient of 0.980 for a Varley and Gradwell correlation and a regression coefficient (Podoler and Rogers, 1975) of 0.8873, over-wintering disappearance was the outstanding key factor. Overwintering mortality governs 88% of the year-to-year fluctuations in the population of P. vulgatissima. Pupal mortality operated in a density-dependent manner (b=0.455), although the regression coefficient indicates that it is working in an under-compensating fashion. Thus pupal mortality would be unable to regulate the population on a year-to-year basis, but after a population perturbation it would be able to return the population to its equilibrium after 2-3 generations. The importance of pupal mortality is therefore uncertain, as SRC willow plantations are harvested every 2-5 years.Any measures taken to reduce the number of overwintering sites available in and around SRC willow plantations should reduce the population of P. vulgatissima and therefore its detrimental effects on the crop. Over 80% of adults overwinter outside the plantation. Particularly attractive are cracks in untreated fence posts surrounding SRC plantations. If the fence posts were treated with a preservative, which prevented cracking, it would severely reduce the number of proximate overwintering sites available for P. vulgatissima, thus reducing their re-invasion success in the spring. As part of the husbandry, small corrugated rolls of cardboard (100 mm x 300 mm) could be stapled to the fence posts. These small overwintering traps can yield up to 600 beetles per week during the winter months. All that need be done is to replace these traps every couple of weeks and dispose of the full ones, thus removing the beetles. This pest control method may be very effective at preventing outbreaks of chrysomelids, particularly at sites with low densities of P vulgatissima.