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Mysore J. Agric. Sci., 51(A): 53-58, 2017 Ecological and Foraging Aspects of the Cabbage Webworm, Hellula undalis Fabricius SIVAPRAGASAM, A. Cabi Southeast Asia Regional Center, Building A19, Mardi, Serdang, Selangor, Malaysia E-mail: [email protected] ABSTRACT The paper addresses the influence of temperature, ascertains the weed hosts and elucidates the foraging aspects of the larva of the cabbage webworm (CWW), Hellula undalis (Fabr.), a major pest of head or English cabbage (Brassica oleracea var. capitata) grown in the Malaysian lowlands. The optimal temperature for the development of CWW was 30ºC, whereby the intrinsic rate of increase rm , was the highest. The ubiquitous weed, Cleome rutidosperma (Family: Capparidaceae) was found to be its natural host. The level of infestation of CWW on C. rutidosperma in the field was low, ranging from 0.1 % to 3.45%. Nevertheless, it provided a source population for the monocrop cabbage causing significant damage. The intra and inter plant foraging studies revealed that the CWW larva preferred the cabbage shoot to the leaf and this preference enhances its survival rate in the field. Keywords: Cabbage webworm, Hellula undalis, temperature, weed host, foraging behavior INSECT pests and diseases are major biotic factors affecting head or English cabbage production both in the highlands and lowlands of Malaysia (Sivapragasam and Loke 1992). Among the pests which inflict serious damage, particularly in the lowlands, is the cabbage webworm (CWW), Hellula undalis (Fabr.) (Lepidoptera: Pyralidae). Although this insect has also been recorded in the Malaysian highlands (Ooi 1979), it has reached serious pest status only on cabbage grown in the lowlands. Stage specific crop life tables on cabbage (Sivapragasam, 1994) suggested that H. undalis is the most important FIGURE 1. Larva of Hellula undalis pest contributing to about 41 percent of total mortality of the plants during the pre-heading stage. Although CWW infests other cruciferous plants, such as chinese mustard, Brassica chinensis (L.), chinese kale, B. oleracea var alboglabra Bailey and radish, Raphanus sativus (L.), it is more serious on head cabbage because damage by a single larva (Figure 1) boring into the growing shoot of the cabbage plant (Figure 2) before its pre-heading stage could result either in the death of the plant or in producing small-sized multiple cabbage heads which are unmarketable. As such, there is no economic threshold level to initiate insecticide treatments but to FIGURE 2. Hellula damage on head cabbage advocate for preventive applications. 54 SIVAPRAGASAM, A., et al. There is also potential for indirect damage due Evaluating weed as hosts of CWW to soft rot, Erwinia carotovora. In any case, complete To ascertain which of the weeds found in or near control of CWW solely with insecticides is fraught the lowland cabbage ecosystem were serving as hosts with challenges due to the webbing behaviour of the to CWW, common broad-leaf weed species were larva and to some extend resistance development examined in situ and also collected and brought to the (Sivapragasam 1994). Thus, to develop pragmatic laboratory to examine for eggs and other stages of management strategies of CWW, an understanding of CWW. Feeding tests were also done under laboratory its biology and ecology in the Malaysian context is conditions with temperature ranging from 24ºC to pertinent. Sivapragasam (2005) described some 28ºC and R.H. 80±5%. Fresh young leaf samples of aspects of its biology and development. To each weed were offered to five first and third instars complement the current repository of knowledge, in of CWW confined in a Petri dish (10.0 x 2.0 cm) lined this paper some aspects of the ecology and foraging with moistened filter paper. First and third instars fed behavior are reported. The aspects are: (i) Effect of on cabbage leaves served as the ‘control’. Further temperature on the development of CWW; (ii) details of the experimental set-up are described in Evaluating weeds as host of CWW and (iii) Dispersal Sivapragasam (1994).For each weed species, the tests and foraging behaviour of the CWW larva. were repeated 3xand in all a total of 19 weed species belonging to 12 families were used in the tests. MATERIALS AND METHODS Dispersal and foraging behaviour of the CWW Here, general details of the materials and larva methods are described. For specific details, refer to The intra- and inter-plant movement of CWW Sivapragasam (1994). larva on cabbage were studied in a glasshouse Effect of temperature on development usingcaged 1.5 month old cabbage plants with about 10 leaves. Details of the experimental procedures are The CWW used in this study was obtained from given in Sivapragasam (1994). Briefly, for the intra- cultures maintained in the laboratory on cabbage plant movement study, a cohort of eggs in an leaves. All experiments were conducted in controlled aluminium egg card (< 24h old) was clipped to the temperature incubators (RumedR) maintained at 3rd leaf of each cabbage plant (the ‘egg leaf’). Leaf different constant temperatures of 10, 15, 20, 25, 30, number 1 is the first open leaf from the top. The 35 and 40 ºC. Temperatures were maintained within number of larva on each of the ten leaves was ±1ºC and the R.H. between 80±5 percent at each observed daily for 10 days. The experiment was temperature studied. The development and survival replicated 10x and data from all ten replicates (or rates of the immatures at each temperature were plants) were pooled for analysis. determined using a cohort of at least 100 eggs laid The inter-plant movement studies were within 24 h. At each temperature regimen, newly conducted using a larger cage (Sivapragasam, 1994). emerged larvae were isolated individually in Petri The design of the experiment was as follows: In the dishes (9.0 x 1.5cm2) and fed with excised young centre of the cage was a cabbage plant (hereafter cabbage leaves until pupation. The longevity and called the source plant) with a cohort of 10 eggs (< oviposition of the adult that emerged from each 24h old). The ‘source’ plant was surrounded by four temperature regimen was determined using a rearing similar-aged cabbage plants placed equidistant from jar provided with leaves of Brassica juncea as the each other and the ‘source’ plant. However, one leaf oviposition substrate. The number of eggs laid and from the ‘source’ plant and that of each of the the adults surviving were recorded for at least 20 pairs neighbouring plants was stapled together to form a bridge between them. A corrugated cardboard paper throughout their life span. The intrinsic rate of sprayed with sticker was used as a barrier to prevent increase (r ) was computed using tables of age m larval movement between plants except via the specific oviposition (m ) and survival rates (L ) for the x x ‘bridge’. Daily observations were made on the source age interval (x) of one day using the analytical method and neighbouring plants for CWW larva. The described by Birch (1948). experiment was terminated when all the larvae has ECOLOGY AND FORAGING OF THE CABBAGE WEBWORM 55 pupated in the pupal tray filled with vermiculite placed suggested that the temperature most favorable for at the base of each cabbage plant. The experiment CWW appeared to be around 30ºC. At this was repeated 10 x and the data from all the replicates temperature rm was the highest. This finding agreed were pooled for analysis. with that of Awai (1958) who reported that the optimal temperature for CWW development was 31º C. RESULTS AND DISCUSSION Messenger (1976)illustrated that for a given environmental condition, rm could be used as a Effect of temperature on development measure of favorability and that the higher the value Table 1 shows the development from egg to adult rm, the more favorable were the environmental emergence (TDP), percent survival (egg to adult conditions within which the population resides. Based emergence), adult longevity for both the female and on this, it can be inferred that the temperature male, oviposition rate per day and intrinsic rate of conditions in the Malaysian lowlands with the mean monthly temperatures of ca 29º C (range: ca 24ºC to increase (rm) of CWW under various temperature regimens. Egg eclosion occurred from 15ºC until 35º 35ºC) is optimal for CWW development. On the other C; none of the eggs hatched at 10ºC and 40ºC. TDP hand, in the cooler highlands(e.g. Cameron Highlands, ranged from 14.64 days at 35º C to about 108.25 days a major cabbage growing area at1,400 – 1,500 meters at 15 ºC and the survival rate from 47percent at 25ºC above sea level with a mean temperature of 18.7ºC to 3 percent at 15ºC. The number of eggs laid (13.4ºC to 24.3ºC) development is impeded. Sachan generally increased with temperature until 30ºC but and Gangawar (1980) reported the concomitant declined at 35ºC. The longevity of the male and decrease in the importance of CWW with increase in female differed significantly between temperatures altitude (with the concomitant decrease in temperature) at which the cabbage is planted. The (Table 1). The rm values increased with temperature to a maximum at 30º C and then decreased. effect of temperature on the distribution of CWW is explicitly shown from its data in temperate countries Temperature affected the survival, development whereby CWW is reported to be major problem only and other life table parameters including number of in summer to late autumn (AVRDC 1978; Shirai and eggs laid and intrinsic rate of increase. This study Kawamoto