Entomology. A.J.M. Carnegie. South African Sugar Association Experiment Station Mount Edgecombe, South Africa ABSTRACT

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1 Entomology ACTIVITY PATTERNS OF MYTIgIMNA PIIAEA HAMPS., ATHETIS IGNAVA GN. AND SIMPLICIA EXTINCTALIS ZELL. (LEPIDOPTERA : NOCTUIDAE), WHICH INFEST TRASHED SUGARCANE FIELDS A.J.M. Carnegie South African Sugar Association Experiment Station Mount Edgecombe, South Africa ABSTRACT Between 1971 and 1976 light trap records were kept of the Noctuids Mythimna phaea Hamps. and Simplicia extinctalis Zel I. M. phaea moths were most plentiful between March and June, with a second, smaller population peak later in the year, and these peaks were followed by field outbreaks of caterpillars. Moths of S. exfinctalis were most plentiful between October and March. Studies of caterpillars caged with sugarcane plants growing through a blanket of dry trash showed that M. phaea, S. extinctalis and Athetis ignava Gn. remained under the trash by day but became active at onset of darkness, returning to the trash at dawn or before. There were some differences in activity patterns and only M. phew fed on the living sugarcane, the other two species feeding on dry trash and associated fungi. In laboratory cultures, larvae of A. ignava frequently ate neither growing cane nor dry trash, although cannibalism sometimes occurred. INTRODUCTION Over a wide area of the South African sugarcane industry it is customary for the cane fields to be trashed; i.e. at harvest the trash is removed unburnt from the crop and spread as a blanket or placed along the interrows as a surface mulch. The caterpillars of at least 7 species of Noctuidae haqe been found in such cane fields, and have been listed with comments on behaviour, responsibility lor economic loss, biological and chemical control2. All the species are nocturnal, and the main crop spoiler was identified as Mythimna phaea Hamps. (also referred to as Cirphis, Leucania and Pseudaletia). The caterpillars rest under the trash blanket by day and, during the first few weeks of cane growth, feed on the young ratoons at night. It is interesting that in other countries very similar Noctuids damage cane, although their mode of attack may be rather different. For example, in Australia "night feeders" which have been identified as Mythimna separata (Walk.) and Pseudaletia convecta (Walk.) have caused considerable damages and may attack mature cane even where trashing is not practised, provided there is some debris under which they can shelter. And in Mauritius. the "army worm" Leucania loreyi Dup. damages young ratoons, but

2 ENTOMOLOGY only where burning has occurred". These species may be considered as belonging to the Leucania-complex which has been discussed in a useful paper by Caloral. In South Africa the three most common species of Noctuids in cane fields were found to be M. phaea, Athetis ignava Gn., and Simplicia extinctalis Zell. Neither of the latter two was noted damaging growing cane. S. extinctalis feeds on the dry trash, and in a heavily infested field it may be heard doing so, and Atlzeti,~ ignava Gn. has been noted feeding on dry trash and on associated fungi. The following notes concern these three species and consist essentially of light trap figures and observations on caged caterpillars. Adult populations MATERIALS AND METHODS Since 1971 numbers of various crop spoilers have been recorded by means of a Robinson light trap3. Initially a trap was established at Mount Edgecombe only, and this formed a unit in the network oi such traps which are run on a co-ordinated basis by a number of countries in southern Africa to record the movements of various crop spoilers. More recently further Robinson traps have been established in the cane-growing area. The traps are fitted with an ultra violet light which is activated automatically by a photo-electric switch. Each day the catch is removed and sorted, records being made of numbers of various moths and beetles. Larval populations Larvae of M. phaea, A. ignava and S. extinctalis were collected from beneath field trash during the day or were taken from growing ratoons in the fields at night. The caterpillars were caged with cane plants growing in plastic trays let into the floors of glass-sided cages (150 x 75 x 40 cm) which were covered with gauze on the top. A thick layer of cane trash was distributed over the floor of the cages which were placed on an open veranda. Numbers active or feeding were noted at intervals during the nights. Since caterpillars were inactive during moulting and before pupation, activity or feeding was expressed as a percentage of the total numbers recorded on any particular night. Total numbers in the cages varied, but were generally between 50 and 300 and, where numbers in the cage were very low, the figures obtained were not included. The nights concerned were dark and there was no moonlight. Cage temperatures were noted, but there was little fluctuation (14 to 21 C). Counts were made with the aid of a dim torch which, for the brief period involved, had no apparent effect on the caterpillars. It has been noted that, if a very bright light is turned on feeding Mythimna, they drop off the plant.

3 A.J.M. CARNEGIE 713 Light trap figures Mythimna phaea RESULTS Monthly averages from the Mount Edgecombe light trap are shown in Fig. 1 a, b, c, d and e, and from the Mtunzini light trap (100 km distant in a north-easterly direction) in Fig. 1 f and g. Each year there was a clear peak in numbers between March and June. This was followed by a period of low numbers, leading to a second, lower peak before numbers dwindled in the summer months. An exception was 1972, when numbers in the second peak were higher than in the first, and remained high during summer. From July 1974, separate records were kept for males and females. Numbers of both sexes followed the same seasonal pattern but numbers of females were higher for all but 2 months, with an average ratio of 2,4 : 1. FIGURE 1. Numbers per night (expressed as monthly averages) of Mythirnr~a phuea in light trap. a, b, c, d and e Mount Edgecombe; f and g Mtunzini ,,,,,,,,,,, J F M A M J J A S O N D MONTH J F M A M J J A S O N D ' MONTH

4 714 ENTOMOLOGY Simplicia extinctalis Records were kept from,august 1972 until April 1975 and are shown in Fig. 2 a, b, c, and d. The seasonal pattern was similar for each recorded year, with a peak in summer and low numbers trapped between March and October. Numbers of Athetis ignava were not recorded in the light trap. Some relationship has been discerned between light trap figures and seasonal patterns of temperature and rainfall, and this is being further investigated. o4 y F) o 'FIGURE 2. Numbers per night (expressed as monthly averages) of Simplicia extinctalis is A MONTH O light trap at ~~ujnt Edgecornbe. Activity patterns of caged larvae Mythimna phaea The following observations were derived in a similar manner to those made earlier2, but numbers active in the cage were distinguished from those actually feeding, as is shown in Fig. 3 and 4. Caterpillars became active before darkness was complete, moving up from beneath the trash blanket onto the cage sides and onto the plants. Some began feeding almost immediately but others wandered for some time before doing so. Largest numbers active or feeding were recorded between and (55.00 hours, after which numbers fell rapidly and, by

5 A J.M. CARNEGIE

6 716 ENTOMOLOGY sunrise, all had re-entered the trash blanket except for one or two parasitised individuals. The maximum number recorded active or feeding at any one time was always considerably lower than the total number known to be present in the cage. Only living cane leaves were eaten. Simplicia extinctalis i I I The activity 'pattern is shown in Fig. 5. Caterpillars became active at dusk, and remained so throughout the night, but numbers visible above the trash blanket fell after hours and continued to do so until dawn. This species did not attack living sugarcane and seemed to live almost exclusively on dry trash, although first instar larvae fed on moist debris and fungi associated with trash. Athetis ignava The activity pattern is shown in Fig. 6. The pattern was similar to that oi Mythimna but the points were more widely scattered. In the cultures it did not feed on living sugarcane at all, although some feeding on the dry trash occurred. In laboratory cultures, when given a choice of growing cane or dry trash, it frequently ate neither, although sometimes cannibalism occurred. DISCUSSION It is interesting that a peak in numbers of Mythimna phaea moths should occur at about the time when the cane cutting season begins, i.e. April to May, and that numbers should dwindle as the cutting season ends at about the end of the year. Whether or not moths are attracted to recently cut cane fields has not been established, but they have been seen feeding on the ends of recently cut stalks. Regular monthly figures for larval populations are not available, but records of outbreaks suggest two waves per cutting season; the first in July and the second in late October or early, November. These correspond quite well with the recorded adult peaks. It is interesting also that these peaks occur at relatively cool times of the year and that during the hottest months numbers are low. With Simplicia extinctalis moths the position is reversed, and very few moths were trapped during the cooler months. It has been suggested that Mythimna moths may arrive in large numbers as a result of migration, as is the case with the army worm Spodoptera exempta (Wlk.), but at present no evidence for this exists.

It was not established whether the consistently larger numbers of female Mythimna trapped were the result of a basic difference in numbers, or the result of females being more readily attracted to

7 It was not established whether the consistently larger numbers of female Mythimna trapped were the result of a basic difference in numbers, or the result of females being more readily attracted to the trap than males. It is characteristic of many Noctuid larvae that they remain inactive during daylight hours and feed at night. That this can be a direct response to light intensity may be demonstrated by covering a culture of Mythinzna with black cloth during the day, which will activate the caterpillars. Since Simplicia and Athetis will eat dry trash, it is perhaps surprising that, at dusk, they left the trash blanket in the cage and wandered about without feeding. Had they all been either mature or parasitised such behaviour might be so explaned, but this was not the case. Athetis could possibly have been seeking some weed on which it normally feeds, but Simplicia occurs in weed-free fields in numbers far too high for it to be a weed feeder., Mention should be made of.an experiment where a blanket of cane trash was spread over an area of barren griund near cane fields infested with trash caterpillars. In the course of a few weeks a considerable population of Simplicia larvae developed in the trash blanket. From notes made regarding the field aspect of infestations investigated it is apparent that these have occurred most frequently on northern or western facing slopes. This may be because ovipositing moths favour the residual warmth from the afternoon sun. Earlier work2 has shown that both Athetis and Simplicia have parasites in common with Mythimna, and where they are present in large numbers they may even serve as a useful parasite source. This, together with their ability to break down the dry trash and their apparent reluctance to attack growing cane, could elevate their status to that of a beneficial insect. I REFERENCES 1. Calora, F.B. (1967). A revision of the species of the Leucania-complex occurring in the Philippines (Lepidoptera, Noctuidae, Hadeninae). Philipp. Agric. 50, Carnegie, A.J.M. and J. Dick. (1972). Notes on sugarcane trash caterpillars (Noctuidae) and effects of defoliation on the crop. Proc. S. Afr. Sug. Technol. ASS. 46, Robinson, H.S. and P.J.M. Robinson. (1950). Some notes on the observed behaviour of Lepidoptera in flight in the vicinity of light-sources together with a description of a light trap designed to take entomological samples. Ent. Gaz Williams, J.R. (1961). In Rep. Maurit. Sug. Ind. Res. Inst. p Wilson, G. et al. (1968). In Rep. Bur. Sug. Ind. Res. Inst. Brisbane p. 60.

718 ENTOMOLOGY ACTIVIDAD TIPICA DE M YTIilMNA PHAEA HAMPS., A THETIS IGNAVA GN. Y SIMPLICIA EXTINCTALIS ZELL' (LEPIDOPTERA: NOCTUIDAE) QUE INFESTAN CARAVERALES CON MALOJA A.J.M. Carnegie RESUMEN Se mantuvo registros de 10s noctuidos Mythimna phaea Hamps.

8 718 ENTOMOLOGY ACTIVIDAD TIPICA DE M YTIilMNA PHAEA HAMPS., A THETIS IGNAVA GN. Y SIMPLICIA EXTINCTALIS ZELL' (LEPIDOPTERA: NOCTUIDAE) QUE INFESTAN CARAVERALES CON MALOJA A.J.M. Carnegie RESUMEN Se mantuvo registros de 10s noctuidos Mythimna phaea Hamps. y Simplicia extinctalis Zell, colectados con trampas de luz entre 1971 y Adultos de M. phaea fueron mas abundantes entre Marzo y Junio, con un pico de poblacibn secundario y menor a1 final del aiio; y estos picos fueron continuados con aparecimientos abundantes de orugas en el campo. Adultos de S. extinctalis fueron mas abundantes entre Octubre y Marzo. Estudios efectuados con orugas enjauladas con plantas de caria, creciendo a traves de una camada de maloja seca, mostraron que M. phaea. S. extinctalisf y Athetis ignava Gn. permanecieron debajo de la maloja durante el dia; pero, reasumieron su actividad al comienzo de la oscuridad, retornando a la maloja al amanecer o antes. Hubieron algunas diferencias en la actividad tipica y solo M. phaea se aliment6 en caria viva; las otras 2 especies se alimentaron en la maloja seca y hongos asociados. En laboratorio, frecuentemente las larvas de A. ignava no comian ni caiia en crecimiento ni maloja seca, observandose, a veces, canibalismo de ellas.

What is insect forecasting, and why do it

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