Soil moisture as mortality factor for cutworms, Agrotis segetum Schiff. (Lep., Noctuidae)

Abstract

The extreme fluctuations of cutworm attacks in Denmark have mainly been related to summer rainfall. In the present investigations the influence on the cutworms was split up and dealt with in the following three ways: Relative air humidity ‐ pilot experiments in the laboratory; rainfall ‐ semifield experiment with sprinkle irrigation of 2 m × 2 m plots; soil moisture ‐ greenhouse experiments using carrots in pots. No indications were found of the effect of the relative humidity or of the effects of falling rain. Soil moisture had a pronounced effect on young cutworms, the mortality of which rose with increasing soil moisture. The experiments showed that young cutworms do not stay permanently on leaves, as described in the literature, but hide in the topmost soil, and this behaviour is disturbed by wet soil. Even though the exact reason for the death of cutworms is still obscure, soil moisture is clearly a major cause of mortality. This information is already used for forecasting of attacks. Improvements may, however, be obtained by building a simple dynamic model when further experimental data are available. Zusammenfassung Bodenfeuchtigkeit als Mortalitätsfaktor bei der Wintersaateule, Agrotis segetum Schiff. (Lep., Noctuidae) Das Auftreten von Erdraupen der Wintersaateule in Dänemark fluktuiert sehr stark. Viele Autoren haben einen Zusammenhang zwischen der Fluktuation und den Niederschlagsmengen gefunden. In der vorliegenden Untersuchung wurde die Bedeutung des Niederschlages in jene von relativer Luftfeuchtigkeit (Pilotversuche), fallendem Regen (Semi‐Feldversuche) und Bodenfeuchtigkeit (Gewächshausversuche mit Möhren in Blumentöpfen) gegliedert. Kein Einfluß konnte seitens der Luftfeuchtigkeit oder der Regenmenge festgestellt werden. Dagegen wurde unter den Jungraupen eine sehr hohe Mortalität gefunden, die von der feuchten Erde verursacht war. Während der Experimente wurde die Beobachtung gemacht, daß die Jungraupen sich nicht ‐ wie früher behauptet ‐ permanent an den Blättern aufhalten. Dieses Verhalten war bei nasser Erde gestört. Obwohl der genaue Grund, warum die Jungraupen sterben, unbekannt ist, kann geschlossen werden, daß steigende Bodenfeuchtigkeit steigende Jungraupenmortalität zur Folge hat. Diese Erkenntnis ist für die Prognose von Erdraupenbefall von Bedeutung. In Dänemark wird sie schon verwendet, um die Schadensgefahr zu veranschlagen, aber es wäre wünschenswert, noch mehr experimentelle Daten zu besitzen, um ein einfaches dynamisches Modell als Grundlage für eine Verbesserung des Prognoseverfahrens entwickeln zu können.

Behaviour of 1st‐ and 2nd‐instar cutworms (Agrotis segetum Schiff.) (Lep., Noctuidae): the influence of soil moisture

“…The difference in feeding frequency might even be bigger than could be revealed with the frequency of soil searching used. This result, obtained over a period of 4 days, corresponds very well with the increased mortality found among A. segetum larvae after about 10 days on moist soil with the same experimental set-up (ESBJERG et al 1986). Therefore a rather simple explanation may be suggested regarding the negative influence of excess summer precipitation on cutworm populations.…”

“…The present investigations show that under favourable dry conditions [BOVIEN and STAPEL (1934), MIKKELSEN and ESBJERG (1981) andESBJERG et al (1986)] 1st-and 2ndinstar A . segetum larvae will hide in the soil both day and night but interrupt the hiding 1-2 times during the day for about 1 h to enter the foliage of the plants and feed.…”

“…segetum stay and feed on the soil surface. ESBJERG et al (1986) surface and on the plants increased a few minutes after irrigation as compared to the situation before irrigation.…”

The significance of shelter for young cutworms (Agrotis segetum)

“…However, the most obvious effect from lack of shelter in the present investigation is the significantly lower mortality (Table 1) of small cutworms which could shelter in the vermiculite. This result corresponds on the one hand with shrinkage as sign of larval starvation after four days without shelter (Esbjerg, 1988) and on the other hand with the increased L~ -L 2 mortality, when there was moist soil around plants for 7-14 days (Esbjerg et al, 1986).…”

“…In the UK (Bowden et al, 1983) and the GDR (Ht~lbert, 1983), summer precipitation was found to be a major regulating factor, and in Denmark (Mikkelsen & Esbj erg, 1981) the numbers of days with precipitation in June and July were found to have a major negative influence on cutworm survival. Esbjerg et al (1986) showed soil moisture to be a mortality factor for young cutworms, but not for older (L 5 -L6) larvae (Esbjerg, in prep.). Further studies on young cutworms revealed that in dry soil, the small larvae leave the soil only for short periods to feed on the leaves of plants during the day (Esbjerg, 1988).…”