This page by: Brent Rowell, Dept. of Horticulture, University of Kentucky, email: browell@uky.edu
Diamondback Moth
Larvae of the diamondback moth (Plutella xylostella, or DBM) cause the most damage to crucifer crops worldwide and the most serious damage to vegetable crops in Southeast Asia. Crucifer crops affected include cabbage, Chinese cabbage, Chinese kale, cauliflower, broccoli, mustard, Chinese radish, pak-choy, and several others. Most DBM populations are maintained below damaging levels in Europe and in the northern United States by effective naturally-occurring insect predators and parasitoids. Lack of effective DBM parasitoids in the tropics and subtropics is considered a critical reason for the persistence of the control problem in lowland Southeast Asia.
DBM control costs (primarily pesticide costs) are estimated to be US $1 billion per year worldwide. DBM larvae cause the most serious damage to crucifer vegetable crops in central Thailand as the pest has become resistant to nearly every class of insecticide. When new and effective insecticides become available, they are used continuously for DBM control until resistance develops and the product is no longer effective. This can occur within one or two growing seasons. The overuse/abuse of pesticides against DBM has become a serious problem in many parts of Thailand. This overuse causes health problems for farmers applying the pesticides, may contaminate soil and water, and may result in excessive residues in vegetables. In addition, natural enemies of DBM and other pests are killed by this over-reliance on pesticides. It is hoped that this brochure will increase awareness of the diversity of DBM parasitoids found in Thailand and stimulate increased efforts in biological control and integrated pest management (IPM).
The life cycle of this pest is illustrated is the diagram below (Figure 1). The larval stages cause the damage to cabbage and other crucifer crops. The usual symptoms of DBM damage are small transparent, papery, "windows" on the surface of leaves; these "windows" do not go all the way through the leaf (Figure 2).
Egg parasitoids
Every stage of the DBM (egg, larval, pupal) is attacked by parasitoids in Thailand. Egg parasitoids are very tiny wasps that deposit their eggs and develop inside eggs of insect pests. At least one DBM egg parasitoid species, Trichogrammatoidea bactrae, is naturally-occurring in Thailand. This parasitoid was mass reared, released, and field tested by the Department of Agriculture in the mid-1980s and 1990s. Parasitism ranged from 16% to 45% of DBM eggs in unsprayed experimental fields in the lowlands. Experimental results have shown that T. bactrae can control DBM in solitary outbreaks but that it is easily killed by insecticides used to control other crucifer pests. Simple mass rearing techniques have been developed for this and other Trichogramma species in Thailand.
Larval parasitoids
Some DBM parasitoids deposit their eggs in only the larval stages of DBM. These are called larval parasitoids. The adult female parasitoid deposits one or more eggs inside the body of the DBM larva; the egg hatches and the parasitoid larva grows inside the DBM larva without killing it. After a while the DBM larva dies and the parasitoid larva makes its cocoon. Some parasitoid species make their cocoons outside the dead or dying body of the DBM larva (as does Cotesia plutellae) while others make their cocoon inside the DBM cocoon (as does Diadegma semiclausum). The adult parasitoid (wasp) emerges from its cocoon, mates, and the cycle repeats itself for as long as DBM hosts are available.
Cotesia plutellae. This species is a small (2.5-2.6 mm, Figure 3) wasp in the family Braconidae. It is most often observed in the field as a tiny white cocoon (3-3.5 mm) attached to leaves of crucifer crops. Sometimes it is found next to the dead body of its DBM caterpillar host as in Figure 4. Its life cycle lasts about 13 days from egg to adult emergence. The adults live much longer (28-35 days) if flowering plants and nectar are available. C. plutellae is the dominant larval parasitoid of DBM in Thailand. Parasitism has been reported as high as 88% but usually ranges around 20-40% of DBM larvae. It has been found during every month of the year in parts of northern Thailand where crucifer crops are grown year-round. It has even been found in some fields in the North where harsh organophosphate pesticides were being used. This suggests that some populations of C. plutellae may have become resistant to pesticides used in certain areas. C. plutellae is also more abundant in areas with overlapping plantings of crucifer crops. It is not unusual to find one or more C. plutellae cocoons per plant in abandoned crucifer plantings in these areas.
Unfortunately, in most cases successful biological control of DBM in lowland Southeast Asia has not been accomplished by naturally-occurring or introduced C. plutellae alone. Although field collections indicating C. plutellae parasitism at or above 50% are fairly common, this usually occurs after DBM populations have already reached damaging levels. Good control is possible, however, when Cotesia is present and safe control products like Bt (i.e., Florbac, Xentari, Bactospeine, etc.) are used. Successful biological control has also been achieved in lowland crucifer fields with this species when multiple releases were made during the growing season. C. plutellae releases within nylon nethouses have also successfully controlled DBM in Thailand. Biological control with C. plutellae or other parasitoids is easier with crops like broccoli or cauliflower compared with cabbage or Chinese kale. Broccoli, cauliflower, or crucifer seed crops can tolerate more leaf damage than cabbage or Chinese kale.
Macromalon orientale. This parasitoid is a wasp in the family Ichneumonidae (4-6 mm long, Figure 5). Not much is known about the distribution of this parasitoid in Thailand. So far this species has been found only in Chiang Mai province where it was reared from both DBM larvae and pupae that were collected from crucifer crops at Maejo University in 1989-90. At that time low numbers of this species were found late in the growing season in February and March and parasitism was only 0.5% to 6% with an overall average of only 3%. M. orientale was also found in abandoned highland cabbage fields at Nong Hoi (elev. 1000 m) in January, 2004. It is considered an important DBM parasitoid in the Assam region of India. M. orientale has never been mass reared and released in Thailand.
Diadegma semiclausum. This parasitoid (5-7 mm, Figure 6) is also a wasp from the family Ichneumonidae. It is best adapted to cooler, temperate climates. Although it does not occur naturally in Thailand, it can and should be mass reared and released here. It has been the most effective of all DBM parasitoids in tropical highland production zones where it has become established after being mass reared and released. D. semiclausum‘s life cycle is 12-16 days from egg laying to adult. Adults live up to 25 days when honey or nectar from flowers is available but survive for only 3 days if no food is available.
This species has been introduced to many Southeast Asian countries over the past decade and has controlled DBM in the highlands of Malaysia, Vietnam, Laos, and the Philippines with subsequent dramatic reductions in pesticide use. This species was imported by the Department of Agriculture in the mid-1990s; however, only a few releases were made in the central highlands (Petchaboon province) where it reportedly did not become established. D. semiclausum has not yet been tried in the North where it can be mass reared and released in the highlands in conjunction with IPM programs of the Royal Project Foundation. If it is not killed by insecticides, it should be successful in controlling DBM in the northern highlands. Farmers can be provided with Bt products (Bacillus thuringiensis)to use as a component of the IPM program where parasitoids are released. Unfortunately it does not survive in lowland areas below about 800 m.
Other larval parasitoids. There are several other larval parasitoids of DBM which could possibly used for biological control in Thailand. These include a temperate species, Microplitis plutellae, which is a small braconid wasp very similar to C. plutellae. Studies in Taiwan have shown that this species is more heat tolerant than other temperate DBM parasitoids; it has been mass reared and released in Cambodia where it contributed to DBM control. Another DBM parasitoid, Oomyzus sokolowskii, was imported, mass reared, and released for DBM control by the Department of Agriculture at Kanchanaburi in the mid-1990s. Parasitism in the field by this species was low and it was concluded that it may not be very suitable for biological control of DBM in Thailand.
Pupal parasitoids
Some DBM parasitoids attack and deposit their eggs only in the cocoons (pupae) of DBM. These are called pupal parasitoids; they do not compete directly with larval parasitoids of DBM. The adult wasp emerges from the remains of the DBM cocoon. Naturally-occurring pupal parasitoids of DBM have been found in Thailand.
Diadromus collaris. This ichneumonid wasp (6-7 mm) deposits its eggs only in DBM cocoons (Figure 7). Its life cycle is 15 days from egg to adult emergence. This species is naturally occurring in Thailand where it has been found in Chiang Mai and Petchaboon provinces. It parasitized 9%-31% (average 16%) of DBM pupae that were collected at Maejo University (elev. 300 m) in February and March of 1990. Many adults of this species were also observed in the field at Maejo in March of 1989-90 and in abandoned highland cabbage fields at Nong Hoi (elev. 1000 m) in January, 2004. Parasitism was 23% and 63% of DBM pupae collected in the Khao Khor highlands of north-central Thailand in February of 1985 and 1986, respectively; 28% of pupae collected in Petchaboon were parasitized by this species in 1995. D. collaris appears to contribute significantly to natural control of DBM and has been reared and released for DBM control in several countries in southeast Asia.
Brachymeria excarinata. This is a very small (about 3 mm), stocky parasitoid in the family Chalcidae (Figure 8). B. excarinata were found in DBM pupae collected late in the growing season in February and March of 1990 at Maejo University in Chiang Mai. Pupal parasitism ranged from 0 to 25% with an overall average of 9%. B. excarinata were also found to be hyperparasitic (i.e., a parasitoid of another parasitoid species) of Cotesia plutellae at Maejo, but at a very low frequency. This species also occurs in some parts of India both as a primary DBM pupal parasitoid (15-60% parasitism) and as a hyperparasitoid of C. plutellae. Brachymeria sp. was more recently reported parasitizing DBM pupae in samples collected by the DOA from Tak province. It was also successfully mass reared by the DOA in the mid-1990s. A single individual of another Brachymeria species, identified as Brachymeria lasus, was reared from DBM pupae collected in January 1990 from Loei province.
Acting alone, none of the pupal parasitoids can match the effectiveness of Diadegma semiclausum; however, together they can have considerable impact on DBM populations. Combined parasitism by parasitoids D. collaris and B. excarinata, for example, ranged from 9 to 56% with an overall combined average of 25% of DBM pupae parasitized during February-March, 1990 at Maejo in Chiang Mai.
Improving biological control of DBM and other crucifer pests in Thailand
Many farmers have never seen any of these DBM parasitoids in their fields. This is usually because they have not been trained to look for them. It is also because the parasitoids have often been killed as the result of overuse of harsh pesticides for DBM control. Farmers can do many things to reestablish and protect natural enemies in their vegetable plantings.
Cultural practices such as irrigation methods can be modified to improve DBM control and reduce pesticide use. In central Thailand, for example, many growers use frequent overhead irrigation which washes off pesticides–including safe products like Bt–soon after they are applied. This results in loss of control and subsequent overuse of pesticide mixtures or "cocktails". Natural enemies have little chance to survive under these conditions. Drip irrigation (Figure 9) is being introduced in some regions and could significantly help improve control of DBM and other crucifer pests. Use of drip irrigation will also result in water savings and will reduce the incidence of foliar diseases. The extension of drip irrigation could be combined with the use of Bt or other "reduced risk" pesticides in conjunction with release of DBM parasitoids. These could be the building blocks of an IPM program for crucifer pests in central Thailand.
Another common production practice which affects biological control by parasitoids is the continuous production of crucifer crops in the central region and elsewhere in Thailand. Although there may be some benefits–such as the continued survival of the host (DBM) and consequently its parasitoids (i.e. Cotesia plutellae)–this practice appears to be for the most part detrimental. Populations of all crucifer pests such as DBM, cabbage looper (Trichoplusia ni), imported cabbage worm (Pieris rapae) cluster caterpillar (Spodoptera litura), and fleabeetles (Phyllotreta sp.) are maintained by continuous cropping and quickly infest new plantings. In contrast, it is often possible to produce a cabbage or other crucifer crop with little pesticide use in areas where the crop was preceded by a "crucifer-free period". This crucifer-free period of 3-4 months will often result in drastic reductions of pest populations. At Maejo University, it is possible to produce a crucifer crop with very little need for insecticide treatments when planted after a rainy season rice crop.
Farmers can encourage parasitoids by planting flowering crops such as "Kwangtung" (Figure 10) or in some cases by not cutting down flowering weeds bordering production fields. Adult parasitoids feed on nectar and growing flowering crops will significantly increase their longevity (and therefore parasitism) in the field. Adults of C. plutellae, for example, live only 5-8 days without food but survive for 28-35 days when allowed to feed on nectar. The same is true for most other parasitoids.
Thailand has several naturally-occurring DBM parasitoids that have not been found in other countries in Southeast Asia. Unfortunately these appear to have been killed off by overuse of harsh pesticides in many areas. The diversity of indigenous DBM parasitoids can contribute significantly to the natural control of DBM, especially late in the dry season. Other treatments will often be required in the early part of the growing season, however, to keep DBM and other crucifer pests below economically damaging levels. Using treatments less damaging to parasitoids such as Bt products or neem will conserve natural enemies and contribute greatly to reducing DBM damage later in the season.
Probably the most effective IPM strategy will be to enhance natural control by reducing or eliminating the use of broad spectrum insecticides which are most damaging to natural enemies. This will require further efforts at IPM education (such as Farmers Field Schools or FFS) which ideally should be coupled with self-sustaining inoculative releases of both indigenous and exotic DBM parasitoids in lowland and highland production zones. The most effective releases will be carried out with the full participation of interested farmers in conjunction with IPM education/extension programs. Areas where successful vegetable FFSs have been or are being conducted should be ideal for the introduction of new biocontrol agents (Figures 11 and 12).
Now that the Department of Agricultural Extension has insect natural enemy rearing facilities all over Thailand, Cotesia plutellae can be reared and released in order to reestablish this species in lowland production regions where its populations may have been drastically reduced by overuse of broad spectrum insecticides. C. plutellae populations may also recover on their own when Bt and other selective control treatments are used on a regular basis. A Cotesia-based IPM program should succeed in Thailand if strongly supported by extension efforts as was the case in the Philippines. Rearing and release of Microplitis plutellae and other parasitoids may also be considered.
Diadegma semiclausum should be re-imported, mass reared and released in the northern highlands, especially in Royal Project-assisted sites at elevations over 800 m. Several researchers and technicians from the Department of Agriculture have already been trained in DBM and DBM parasitoid rearing and Royal Project-trained IPM/Plant Protection advisors could assist in this project. This would be an ideal program for the Royal Project Foundation together with the Department of Agricultural Extension’s biological control centers.
Provided Bt and other selective products are used and parasitoids become established, it should only be necessary to rear and make inoculative releases of DBM parasitoids over a 3-5 year period. Conservation of indigenous natural enemies together with further development and use of exotic species will help alleviate growing public concerns about the environment, farmer safety, and excessive pesticide residues in vegetable crops.
Note:
- DBM life cycle (Figure 1) courtesy of the Department of Agriculture
- Diadegma semiclausum photo (Figure 6) courtesy of Dr. Merle Shepard
- FFS photos by Hein Bijlmakers (Figure 11 and 12)
- all other photos by Brent Rowell.