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N. S. Talekar, S. T. Lee, and S. W. Huang Asian Vegetable Research and Development Center (AVRDC), Shanhua, Tainan, Taiwan, ROC
Abstract
Monitoring of diamondback moth (Plutella xylostella L) population on cabbage and Chinese cabbage over the past eight years has indicated that heavy mans from June to September limit the population of the insect. Therefore, during the dry period when diamond- S back moth is a serious pest, water was applied by sprinkler system placed 1.5 m over the cabbage crop rather than by traditional furrow irrigation. Sprinkler irrigation applied for five minutes at dusk on alternate days over the first three to four weeks, and every day there after, significantly reduced the diamondback moth infestation and increased the yield over drip irrigation check plots which received an equal amount of water.The physical disruption of flying activity, oviposition, and to some extent wash-off of larvae and adults were presumably the major causes of the observed effects. In an intercropping study, cabbage was planted between two rows of each of 54 selected crops intercropping with tomato, dill, garlic, safflower, oat, and barley reduced diamondback moth damage to cabbage. Laboratory study indicated that application of tomato leaf extract to cabbage significantly reduced diamondback moth oviposition on treated surfaces.
Introduction
Diamondback moth (DBM), Plutella xylostella L (Lepidoptera: Yponomeutidae), is a serious pest of cruciferous vegetables in Taiwan and several other countries of southeast Asia, especially around large cities and in specialized vegetable growing areas that supply vegetables to cities throughout the year. This insect is cosmopolitan and reproduces under extremely varied climatic conditions (Paramonov 1953). Although a temperature range of 17°C to 25°C is considered optimum for the pest (Atwal 1955), it is one of the most serious pests of cabbage even in Polar regions of the USSR (Kutsenin 1977). It breeds all year round in warm humid tropical regions. In Taiwan, it breeds throughout the year and can have up to 20 generations per year with considerable overlapping of generations in the field.
At present, farmers in Taiwan and elsewhere in Asia use large quantities of insecticides to control this pest. The insecticide use is intensive, as a result of which the insect has become resistant to several insecticides belonging to all classes of commercially available chemicals. Indiscriminate insecticide use has eliminated many parasites and predators and some previously minor pests, such as Spodoptera exigua in Thailand, have become major ones. This has necessitated the use of alternative control measures which can be used alone or integrated with certain other methods.
At the Asian Vegetable Research and Development Center (AVRDC) our initial research efforts involved finding the sources of resistance with the aim of breeding of Chinese cabbage cultivars resistant to this pest. However, despite intensive efforts. we did not find any source in Brassica campestris germplasm with a high enough level of resistance to initiate a resistance breeding program. We therefore switched our research efforts to identifying suitable cultural practices such as irrigation and intercropping which, when coupled with our on-going pest population monitoring designed to help in forecasting pest infestations, will help in integrated management of DBM.
DBM Monitoring
Accurate information on the seasonality of the pest, and of the crop growth stage at which insect populations will most damage a crop, is critical to the implementation of control measures. Such information, coupled with meteorological data, also helps in forecasting insect pest epidemics. With this purpose in mind we initiated a detailed insect pest monitoring program for Chinese cabbage and cabbage in 1976. Once every month, from January 1976, four-week old seedlings of Chinese cabbage and cabbage were transplanted onto the top of four 30 x 1 m beds, two adjacent beds for each crop. The crops were raised according to standard cultural practices, except that no insecticide was applied. At harvest the land was rototilled, and used again for planting in an identical manner. Infestations of DBM and other insects were recorded from a 30-plant sample once every two weeks starting four weeks after transplanting. The number of DBM larvae and pupae were recorded from each of the 30 plants. Records of daily maximum and minimum temperatures, relative humidity, rainfall and solar radiation were maintained (Talekar and Lee 1985).
DBM infested Chinese cabbage and cabbage from October to May but was practically absent from June through September (Figure l). In 1980, when rainfall was less than one third of normal during the traditional May through September rainy season, the DBM population was high almost throughout the year. The cool dry winter is a crucifer season in Taiwan, and host plants, including wild crucifers, are readily available. The absence of DBM during the summer months, despite the availability of host plants, in our study appears to be due more to frequent rains than to high temperatures. Certain studies (Chen and Su 1978, Chin 1973, Hsu and Wang 1971, Schmutterer 1977) indicate that DBM is capable of multiplying and causing damage at temperatures approaching those of the Taiwan summer. Besides, in Indonesia, the Philippines, and Thailand, where summer temperatures are as high as those in Taiwan but where these temperatures are coupled with dry (rainless) weather, DBM is a particularly serious pest in this season. Therefore, it seems that frequent rains limit incidence of DBM. The larva of this pest is a surface feeder and is frequently washed away or drowned in the cavity created as a result of its peculiar feeding habit. Rain can also disrupt the flying of the adults and thus hamper their movement, including possibly oviposition, and this may also limit infestation. This hypothesis is further supported by the fact that in 1980, when Taiwan experienced severe drought, DBM was present throughout the year. Harcourt (1963) in Canada has also reported rainfall as a major mortality factor in the population dynamics of DBM.
Modification of Irrigation for DBM Control
The study of the possibility of modification of irrigation method to control DBM was the direct consequence of our DBM population monitoring, which indicated that frequent rains during summer adversely affect the DBM infestation of cabbage and Chinese cabbage (Talekar and Lee 1985). Two experiments were conducted during 1983-84 in which the method of irrigation was modified. In the first experiment, we used three irrigation methods: 1. Furrow irrigation check where water was confined in furrows on both sides of raised beds on the top of which cabbage was transplanted; 2. Sprinkler irrigation where water was sprinkled over the plot from one central point where a Rainbird-type sprinkler was established; and 3. Sprinkler irrigation where the water source, a perforated plastic pipe, was fixed 1.5 m above cabbage plants in such a way that when filled with pressurized water, tiny water jets sprinkled over the plants.
Six weeks prior to transplanting, two rows of cabbage were transplanted all around the field and infested with a large number of DBM adults. This infestation later acted as the infestation source for the experimental plots.
In the furrow irrigation system, the plots were irrigated when necessary as judged by the dryness of the soil. In both sprinkler irrigation treatments the plots were irrigated for five minutes each at dusk once every two days for the first three to four weeks and every day thereafter. All other cultural practices such as weeding, fertilizer application, prophylactic disease control measures and so on were observed but no insecticide was applied.
The crop was evaluated for DBM infestation twice when the insect population in the furrow irrigation plot was high. The numbers of DBM larvae and pupae were recorded on 20 randomly selected plants from each plot. At harvest, yield was determined by weighing the cabbage heads after removal of non-wrapper leaves.
The results (Table 1) indicated that both sprinkler systems of irrigation significantly reduced the insect infestation and increased the crop yield.
Besides insect infestation, the irrigation system probably played a significant role in other ways in increasing the yield. The frequent sprinkler irrigation kept the soil moist most of the time providing adequate moisture all the time, as against furrow irrigation where water was applied, based on visual observation, only when judged necessary. This probably induces water stress at certain periods, and perhaps adversely affects yields.
In the second experiment, therefore, instead of furrow irrigation we used a drip irrigation check. In this case, the frequency of application and the amount of water received by the sprinkler and drip irrigation plots were maintained equal. The sprinkler irrigation was modified slightly so that water was delivered from above the plants by a perforated rotating pipe instead of a fixed one. All other management practices, irrigation timing and frequency, insect infestation, and observation methods, remained the same as in the first experiment.
The results of this experiment are summarized in Table 2. The numbers of DBM larvae and pupae were consistently lower in the sprinkler plots than in the drip irrigation plots during each observation. The numbers of marketable heads and total yield in the sprinkler plots were significantly greater than in the drip irrigation plots. On average, cabbage heads in the sprinkler irrigation plots weighed 738 g as against only 421 g in the drip irrigation plots. Since the amount and frequency of water received was equal in both treatments, the greater yield from sprinkler plots must have been due to lower DBM infestation.
Sprinkler irrigation presumably drowns and washes away DBM larvae feeding on the leaf surface. It also disturbs the adult moths and forces them to fly upon which the water droplets wash them away. Since sprinkler irrigation was carried out close to dusk, when the DBM mate and start laying eggs (Harcourt 1957), there seems a distinct possibility that this treatment disturbs mating and/or oviposition.
Intercropping for DBM Control
The use of modern high yielding crop cultivars creates monocultures with a narrow genetic base which are subject to increasing losses to pests. This is because monoculture reduces the diversity of pest species, which tend to explode in numbers because they have a greater potential for building up their numbers under conditions of reduced competition. In many specialized vegetable producing areas in Asia, crucifers are grown year-round, and this provides a year-round source of food for DBM and other crucifer pests. This factor, along with DBM's development of resistance to insecticides, has played a significant role in DBM becoming an important pest.
On the other hand, intercropping is a common practice with other crops in many parts of Asia. Under certain combinations, intercropping has beneficial effect in reducing insect pest damage (Nickel 1973, IRRI 1974, Buranday and Raros 1973, Karel et al 1982). However, such intercropping is rarely practiced with crucifers, especially in specialized vegetable production areas where DBM is a menace.
The purpose of this experiment, therefore, was to explore the possibility of intercropping cabbage with other crops to reduce DBM damage to cabbage.
Seeds of 54 crops were planted one crop/plot, in two lines, 40 cm apart, on the top and along the length of 10 m long and 1.5 m wide plots. Each crop was planted in three randomly arranged beds, each bed representing one replicate. One month after sowing of intercrops, four-week old cabbage seedlings were transplanted in a single row at the center along the length of each plot. The crop was raised by customary cultural practices except that no insecticide was applied. At 40, 60, and 80 days after transplanting, DBM infestation of cabbage was recorded by counting the number of larvae and pupae/10 plants in each plot.
The 54 crops that were planted along with cabbage were as follows: amaranthus (Amaranthus blitum), barley (Hordeum vulgare), barnyard millet (Echinochloa frumentacea), barseem (Trifolium alexandrinum), broccoli (Brassica oleracea var italica), Brussels sprouts (Brassica oleracea var gemmifera), buckwheat (Fagopyrum esculentum), carrot (Daucus carota), cassava (Manihot esculenta), cauliflower (Brassica oleracea var capitata), celery (A plum graveolens), cabbage (Brassica oleracea var capitata), chill) pepper (Capsicum annum), Chinese cabbage (Brassica campestris ss p pekinensis), coriander (Coriandrum sativum), corn (Zea ways), cosmos (Cosmos sulphureus), cucumber (Cucumis sativus), daisy (Bellis perennis), dill (Anethum graveolens), eggplant (Solanum nigrum), four o'clock (Mirabilis jalapa), garlic (Allium sativum), hollyhock (Althaea rosea), kale (Brassica acephala), kodo millet (Paspalum scrobiculatum), Leek (Allium porrum), marigold (Tagetes erects), mesta (Hibiscus cannabinus), methi (Trigonella foenumgraecum), mustard (Brassica juncea), oat (Avena sativa), pal tsai (Brassica campestris ssp chinensis), parsley (Petroselinum hortense), pea (Pisum sativum), portulaca (Portulaca grandiflora), pumpkin (Cucumis pepo), radish (Raphanus sativus), rape (Brassica napes), ridge gourd (Luffa aculangula), rutabaga (Brassia napobrassica), safflower (Carthamus tinctorius), shallot (Allium ascalonicum), smooth gourd (Luffa cylindrica), sorghum (Sorghum bicolor), soybean (Clycine max), spinach (Spinacia oleracea), sugarcane (Saccharum officinarum), sweet pepper (Capsicum frutescens), sweet potato (Ipomoea batatas), tobacco (Nicotiana tabacum), tomato (Lycopersicon esculentum), white potato (Solanum tuberasum), and wild tomato (Lycopersicon hirsutum f glabratum).
Among the intercrops, cabbage planted between barley, dill, garlic, oat, safflower, or tomato had relatively less DBM larvae and pupae, especially during the third observation when insect population was especially high (Table 3). It must be pointed out that due to wide variation among replicates, the results were not statistically significant. Crops like safflower, oat, and barley grew tall and such crops act as a barrier against DBM which is carried by winds over long distances (French and White 1960). Such crops also by their shadowing effect, adversely affect growth of cabbage plants and hence have limited utility. On the other hand dill, garlic, and tomato do not grow tall and their influence in reducing DBM damage to cabbage appears to be a characteristic repellent odor that each of them possesses. Some studies have indicated the useful effect of tomato intercropping in reducing infestation of crucifers, mainly cabbage, by insect pests (Buranday and Raros 1973, Vostrikov 1915, Srinivasan 1984). Studies have also shown that certain principles in tomato leaf extract adversely affect oviposition of DBM on cabbage and Chinese cabbage (Gupta and Thorsteinson 1960, AVRDC 1985). It is possible that similar principles in dill and garlic repel the DBM infestation. Studies with large plot size are under way to modify the cultural practices so that both main crop and intercrop can be grown more profitably with reduced DBM damage to cabbage.
AVRDC. 1985. 1984 Progress Report. Asian Vegetable Research and Development Center, Shanhua, Taiwan, ROC (in press).
Buranday, R. P. and R. S. Raros. 1973. Effects of cabbage tomato intercropping on the incidence and oviposition of the diamondback moth, Plutella xylostella (L.). Philipp. Entomol. 2:369-374.
Chen, C. N. and W. Y. Su. 1978. Influence of temperature on the development and feeding amount of diamondback moth larvae on cauliflower. Plant Prot. Bull (Taiwan). 20:224-231.
Chin, D. 1973. Population fluctuation of diamondback moth in relation to environmental factors. (in Chinese). Taiwan Agric. 10:81-84.
French, R. A. and J. H. White. 1960. The diamondback moth outbreak of 1958. Plant Pathol. 9:77-84.
Gupta, P. D. and A. J. Thorsteinson. 1960. Food plant relationship of diamondback moth, Plutella maculipennis (Curt.). II. Sensory relationship of oviposition of the adult female. Entomol. Exp. Appl. 3:305-314.
Harcourt, D. G. 1957. Biology of the diamondback moth, Plutella maculipennis (Curt.) (Lepidoptera:Plutellidae), in eastern Ontario. ll. Life-history, behavior, and host relationship. Can. Entomol. 89:554-564.
Harcourt, D. G. 1963. Major mortality factors in the population dynamics of the diamondback moth, Plutella maculipennis (Curt.) (Lepidoptera:Plutellidae). Mem. Entomol. Soc. Can. No. 32:55-66.
Hsu, S. C. and C. J. Wang. 1971. Effect of photoperiod and temperature on the development of the diamondback moth, Plutella xylostella L. and the parasitism of the braconid, Apanteles plutellae Kurd, NTU Phytopathol. Entomol. 1:3-36.
IRRI. 1974. Multiple cropping. pp 16-34. In Annual Report for 1973. International Rice Research Institute, Los Banos, Philippines.
Karel, A. K., D. A. Lakhani, and B. J. Ndunguru. 1982. Intercropping of maize and cowpea: Effect of plant populations on insect pest and seed yield. pp 102-109. In C. L. Keswani and B. J. Ndunguru: (Eds.) Intercropping: Proceedings of the Second Symposium on Intercropping in Semi-Arid Areas, held at Morogoro, Tanzania, 4-7 August 1980. International Development Research Centre, Ottawa, Canada.
Kutsenin, B. A. 1977. Protection of cabbage in the Polar region. (in Russian). Zashch. Rast. No. 6:22.
Nickel, J. L. 1973. Pest situation in changing agricultural systems: A review. Bull. Entomol. Soc. Amer. 19: 136- 142.
Paramonov, S. 1953. The principal pests of oil seed plants in Ukraine. (in German). Z. Angew. Entomol. 35:63-81.
Schmutterer, H. 1977. Pests of tropical crops. pp 237-242. In J. Kranz, H. Schmutter, and W. Koch (Eds.). Diseases, pests, and weeds in Tropical Crops. Verlag Paul Parey, Berlin and Hamburg.
Srinivasan, K. 1984. Visual damage thresholds for diamondback moth, Plutella xylostella (Linnaeus) and leafwebber, Crocidolomia binotalis Zeller, on cabbage. Ph.D. Thesis, University of Agricultural Sciences, Hebbal, India. 166 pp.
Talekar, N. S. and S. T. Lee. 1985. Seasonality of insect pests of Chinese cabbage and common cabbage in Taiwan, Plant Prot. Bull. (Taiwan). 27:47-52.
Vostrikov, P. 1915. Tomatoes as insecticides. The importance of solanaceae in the control of pests of agriculture. (in Russian). Novotcherkassk. 10:9-12.
Discussion
A. SAVAPRAGASAM: Do you know the reason why, although there is high oviposition on PI234599 cauliflower compared to the other varieties, the number of larvae tends to be low? Is this due to poor egg viability?
C. J. ECKENRODE: No, the egg viability was good on all varieties in these tests. The reason that few larvae are found on P1234599 cauliflower grown in the field is that the larvae either leave the plants because they are unacceptable or are killed shortly after they hatch.
R. S. REJESUS: You observed that light and N influenced resistance inside the greenhouse as opposed to outside. Any possibility that either light or nitrogen is an immediate mediator for the loss or enhancement of resistance?
C. J. ECKENRODE: This has not been determined with certainty, but Dickson and I believe that antibiosis is present in the glossy lines when grown in the field. It is quite possible that light and/or nitrogen influence this.
T. H. CHUA: Could you comment on the taste and quality of the resistant varieties? C. J. ECKENRODE: The taste is perfectly normal, the quality of the cabbage is quite good, but the color may be foreign to some consumers. I do not see any problems with cauliflower.
T. H. CHUA: Why do you think the female DBM prefers to lay eggs on the resistant varieties?
C. J. ECKENRODE: Because of the tactile feel of the smooth leaf or because of some difference in the chemical composition. We are trying to get information on differences in its chemical make-up.
N. WILDING: Speakers have spoken about longevity of different stages in the lifecycle of DBM and about its fecundity. I assume that this work has been done with one or more local strains of the pest. Has any of the speakers studied these factors in strains from elsewhere? How genetically homogeneous in this insect?
C. J. ECKENRODE: It is most probable that such differences do exist.
T. H. CHUA: Do you anticipate the development of biotypes in DBM similar to those of brown planthopper which broke down the resistant rice varieties?
M. H. DICKSON: It is possible, but that is why we also have a breeding program based on multiple gene resistance which would be harder for a biotype to overcome. The breeding program is also more complicated.
G. S. LIM: Your work shows potential for the use of resistant cultivars to suppress
DBM population. Are you aware of any case where crucifers resistant to DBM are cultivated on a fairly large scale?
M. H. DICKSON: No. We intend to test these materials in large plots in 1986.
J. HOFFMANN: Did you obtain glossiness in leaves of DBM resistant cabbage by genetic manipulation or by selection?
M. H. DICKSON: The PI 234599 which had resistance to DBM has glossy leaves. Not all lines with glossy leaves are resistant, only those with the gene for glossy leaves from PI 234599. There are a number of other genes which confer glossy leaves.
B. ROWELL: We grew a cabbage cultivar, Cornell line from Peto Seed Co, which has glossy leaves. But the damage to this line was not less than others. Could it be due to much higher insect population pressure?
M. H. DICKSON: Young plants appear to be quite susceptible, but develop resistance as they mature. This was observed in trials in the Philippines and Australia. There is also some difference in levels of resistance in lines with the glossy leaves, we observe this regularly and it was also apparent in the Philippines. I do not know which of these lines I released was tested by Peto, but I know one was not very resistant to DBM. The lines were released as resistant to lepidopterous pest, more specifically to Pieris rapae and Trichoplusia ni at the time of release. Since then we have done more selection for tolerance to DBM.
G. S. LIM (COMMENT): We also have done some studies on the effect of tomato extract on DBM and would like to share our findings here. We found that tomato extract (1 g ground up leaves in 1000 ml water) will cause reduction in oviposition by DBM adults but has no significant effect on the development and survival of DBM larvae that may be already present on the plants.
T. R. OMOY: You showed the beneficial effect of tomato intercropping on DBM control. Is there any specific cultivar requirement for tomato to be effective? Would tropical tomato do?
N. S. TALEKAR: There is no specific cultivar requirement for tomato to be effective. We used heat-tolerant, heat-sensitive and even wild tomato (Lycopersicon hirsutum f glabratum) and all gave equally beneficial effect as far as deterring the DBM adults from laying eggs is concerned.
M. P. FERINO: Would a lower plant population of the intercrop have the same effect on the DBM as the normal plant population?
N. S. TALEKAR: We have not yet studied this aspect. It would be the aim of a future study to lower the intercrop population to the bare minimum required for DBM control since some of the intercrop species are not economically important and we do not want to waste land under such crops.
T. H. CHUA: What. would be the cost of installing the sprinkler system. I think the cost of US$1000 per 0.2 ha is too high for small farmers to adopt.
N. S. TALEKAR: Yes, the initial cost is definitely too high for small farmers to utilize this system. However, this system can be used over several cropping seasons. This, combined with the savings on insecticide cost and labor cost, will make the system economical. In the meantime we are trying to modify the system to make it less costly as well as more effective.
E. D. MAGALLONA: Usually, with high moisture, such as under sprinkler irrigation, development of diseases is encouraged. Was there any noticeable difference in disease incidence between your check and sprinkler irrigation treatments?
N. S. TALEKAR: NO, we did not observe any unusually high incidence of disease with cabbage. I am, however, afraid that the disease problem will be important if we use cauliflower or broccoli.
L. C. CHANG: What is the present situation in the use of parasites for DBM control in Taiwan?
N. S. TALEKAR: Certain parasites were introduced in Taiwan specifically to control DBM about 10 years ago but the parasites did not get established, possibly due to excessive use of insecticides. Nothing was done after that. At the moment AVRDC has imported Diadegma eucerophaga from Indonesia and we are mass rearing it for release on farmers' fields starting autumn 1985.
T. MIYATA: Your overhead sprinkler system shows promising results as far as DBM control is concerned. Does this method also control other insects such as cabbage looper and cabbage butterfly?
N. S. TALEKAR: During the experimental period we did not get cabbage looper infestation at all and cabbage butterfly population was too low and too unevenly distributed to get any realistic information on its control. However, sprinkler irrigation significantly reduced aphid population. But aphid infestation was rather low and came during late growing stage and its reduction could not have contributed to increased yield.
A. SIVAPRAGASAM: Did you observe any direct effect of overhead sprinkler irrigation on larval mortality, egg mortality, and adult behavior in the field?
N. S. TALEKAR: No, we merely recorded the number of DBM larvae and pupae and yield in both control and overhead irrigation treatments. We did find though that during application of sprinkler irrigation DBM adults flew around and some of them were caught under the water jets and washed away.
B. POWELL: Do you see crucifer tomato intercropping in Taiwan?
N. S. TALEKAR: It is not common. Crucifers are grown in specialized vegetable production areas for domestic fresh market use and export, whereas tomato is mainly grown for processing in different areas. The farmers who grow these crops are different. I have, however, observed crucifer- tomato intercropping in North Sumatra where both crops are grown for the fresh market.
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