In India, more than 50 insect species have been reported on Parthenium, but none of the indigenous insects was found host-specific yet. Based on well documented success by Mexican beetle, Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae), in other countries where they were introduced, beetle were imported from Mexico to India . After in-depth labor atory and field studies, it was found host specific, which can eat only Parthenium, hence, its’ use was permitted by Government of India. Therefore, Mexican beetles can be multiplied and released anywhere in India for Parthenium suppression .
Both the grubs and adults of Z.bicolorata feed on Parthenium leaves.Beetles are off white or light reddish in colour with dark brown longitudinal markings on the elytra, measuring about 6 mm in length. Light yellow eggs are laid generally on ventral side of the leaves and hatch in 4-7 days. There are four instars. The grubs feed for 10-15 days on the leaves and on maturity enter into soil and pupate below up to 15 cm depth. Beetles emerged after 8-12 days. The beetle completes its life-cycle in 22-32 days. Insect completes 5-6 generations under field conditions. The female can lay up to 2500 eggs during its life span.
Both adults and grubs are capable to feed on Parthenium leaves. Grubs after hatching, starts to feed on soft growing leaves which on maturity prefer mature leaves. Adults also feeds on leaves. On well establishment after its release in Parthenium infested area, it may cause large scale defoliation of Parthenium. Continuous defoliation of Parthenium brings reduction in seed bank and restoration of other vegetation.
In general Z. bicolorata remained most active during July to September except a few exceptions where beetle had caused appreciable damage on Parthenium, during February to April at some places in patches near the good moisture regime. At many places, all the stages of beetles may be recorded from the field in extreme winter and summer season near the high moisture regime.
The defoliation of Parthenium is of population dependant. Population build-up is also dependant on rains and temperature. After rains in June-July,population build-up starts but long dry spell can reduce the population build-up of the beetle drastically. Continuous or intermittent rains during June to September results high defoliation in large area by the end of August but dry spell of 15 to 20 days during June-July may result same defoliation by the end of September. Scanty rains or drought conditions during rainy season may cause severe setback in population build-up of beetle hence poor defoliation of Parthenium. Continuous attack of bioagent on the same site for 3- 4 years results restoration of lost biodiversity.
Drastic reduction in flower production of second and third flush of Parthenium during rainy season is brought about by gregarious feeding by the early larval stages of the in sect on the terminal and axillary buds. This feeding does not allow growth of the young plants and they are nipped in the bud.
Mexican beetle does not eat flower directly but in attempt of chewing soft tissues below the flower, it cut the flower from the pedicle hence contribute in destruction of flowers also. It was found that 5 beetles released in caged having flowered Parthenium plants, cut 25-37 (mean 31) flowers out of 115-137 (mean 119.8) within 10 days after their release. Thus one beetle directly cut a mean of 6.2 flowers within a 10 days and the life expectancy of adult beetle may vary from 30 to 150 days.
After continuous attack of beetle for 3-4 years, maximum seed bank is exhausted and other vegetation start to take the niche vacated by the Parthenium. But still, Parthenium germinatesamidst other vegetation from the remaining seeds or seeds come from out side. Mexican beetle defoliate these plants completely and thus further contribute in seed bank reduction.
In a conservative estimate, the beetle controlled 200-hectare land infested with Parthenium within three years of its release at Jabalpur. The cost of most effective herbicide metribuzin for one time application for 200 hectares accounted to be about Rs 5,40,000/-. It is also to be noted that during rainy season, about 70-80% Parthenium germinates at different time after commencement of rains. Hence, at least two applications are required to control Parthenium which might have costed Rs 10,80,000/- in a season. By fourth year of release, beetle was estimated to control 900 -hectare land that amounted to be worth of Rs 2.43 million of herbicide. If the same area has to be removed manually or mechanically, it will be about three times more of the herbicide cost. Therefore, it was concluded that biological control through Z. bicolorata has great potential at least in higher rainfall areas to manage Parthenium. The economic benefits will increase many-folds, if we take into consideration the indirect benefits derived in the form of environmental safety and increase in people health.
Collection from established sites :
By virtue of release in different parts of India, Mexican beetles has established in many parts of India. Therefore, it can easily be collected from the established sites during July to September. For spotting its presence in the area, close observations during rainy season is required by turning the leaves. A new person while trying to collect the beetles from the infested plants, may experience that as soon as he tries to catch the beetle, it falls on the ground. Therefore, it is easy to collect the beetle by placing polythene or containers below the resting site of the beetle. A gentle jerk on twig will be enough to dislodge the beetle directly in to the polythene or container. Collection can be made in ordinary polythene bags or plastic containers that are perphorated with a needle for providing aeration. Upper twigs of plants without leaves should be placed inside the polythene to avoid the shrinking and to provide resting places for the insect.
Selection of release site :
Care must be taken to make the initial releases in undisturbed areas, where manual and chemical control operations are not used. Initial release should be avoided in cultivated land because ploughing of land may disturb the pupation process hence poor survival and subsequent poor establishment. However, augmentative releases in the cultivable field can be made to enhance the population build-up of the beetle after initial establishment. Low lying areas prone to water logging should also be avoided because pupation takes place in soil. Attempts should be made to release the beetle on small and succulent growth of Parthenium. Beetle should never be released on flowering and large size Parthenium plants. If beetles are released at inappropriate site, breeding and population build-up will be slow, hence establishment of the bioagent will be delayed in such sites. Introductory releases in new area may be done involving people participation.This will help to make aware people about the bioagent.
Time of release :
The ideal time for carrying out releases is after the commencement of the rains during rainy season. During that time plenty of succulent Parthenium plants are available in nature. There is no benefit in undertaking releases between Novembers to May when they normally do not breed. However, beetles can be released in dry season also in those sites where sufficient moisture allows the continuous germination of new Parthenium. Such sites may provide suitable microclimate for the beetle to multiply.
Method of field releases :
Adults collected from the multiplication cages or the established sites can be released by just scattering the adults on Parthenium plants. It will be ideal to release full-grown grubs too. Since they enter the soil directly, the chances of their moving away can be avoided. When adults emerge they will feed and oviposit in the same area.
Number of beetles to be released :
Sufficient numbers should be released to increase chances of breeding and thereby ensuring establishment. One adult was found to bring about defoliation of a single Parthenium plant in 6-8 weeks. Therefore, if releases are to be carried out at this rate, about 0.4. to 0.7 million insects will be required per hectare, as in general the weed density varied between 40 to 70 plants per square meter. In practical, it is neither possible nor necessary to release so many insects as they are capable of multiplying rapidly. Releases of about 500-1000 beetles can bring about establishment and eventual control. Once plants are eaten up in the release spot, the insects migrate in to adjacent areas. Taking this into consideration a number of release spots can be selected in a particular place or city, which can act as a focal point.
More releases mean quicker establishment of the beetle and therefore, better control. So, do as many releases as affordable during first couple of years of introduction and make additional releases in isolated areas in future. This method reduces the time for the beetle to build up the population and help the beetles to disperse fast. The least affordable approach is to introduce one or two releases into infested area and do nothing more. This method will get a colony started, but will be slow in terms of time and area.
What happens to the insects after the weeds are eaten?
Parthenium will never be eradicated in a vast country like India due to its immense reproductive and survival capabilities. Some plants will always escape from the attack, which will allow the insect population to sustain itself during years of low weed density.
Can herbicides be used with beetle?
Yes, herbicides can be used ,but with care. Herbicides can be integrated as for most of the cases, herbicides may not kill the beetle population drastically but a few herbicides can kill the Z. biclorata by direct hit. Therefore, only recommended dose should be applied on first flush after initials rains. On observing good population of beetle on the infested site, chemical spray should be avoided.
For multiplying the beetle on small scale, we can use plastic jars/buckets, beakers etc. A bouquet of succulent Parthenium leaves with twigs should be made. To keep the leaves fresh at least for one week, the lower portion with twigs can be kept in wet cotton swab. In small containers we can keep a single pair of male and female while in large containers 3-4 pairs can be kept. Larvae will hatch in 2-3 days from the eggs and will start to feed on the leaves. After a week or on need, old leaves should be changed with fresh leaves. Small larvae should be transferred gently on the fresh leaves with the help of brush. After 12 to 18 days, larvae will be matured. At this time they need soil to pupate. Therefore, a few jars should be made exclusively for pupation purpose by filing wet soil in the containers/jars. The soil can be gently compacted. 4th stage larvae can be transferred in these pupal chambers wherein Parthenium leaves can also be kept as food. After maturity of 4th stage, larva will dig the soil and pupate. After 5-8 days, adult in the form of beetle will be emerged from the soil by making a circular hole. This method is cheap but cumbersome and requires continuous attention. From one such containers, about 50-60 beetles can be obtained in a period of two month.
The most easiest and economic way to mass multiply the beetle is in mosquito cages. The cages may be made of different size as per need and space. Parthenium can be grown in these cages either from the seeds or by transplanting small Parthenium plants from the infested place. After establishment of sufficient Parthenium plants in the cages, about ten pairs of beetles can be released in the space of 6x6 feet. Soon females will start to lay eggs and life cycle of the beetle will start inside the cages. Old and eaten Parthenium plants should be replaced regularly with the fresh plants as and when required. This method require less attention From one 2x2 feet space, about 400- 800 beetles can be obtained in a period of two month. In a net house of about 10x20 m, 10,000 to 15000 beetles can be obtained within a period of two months.
Augmentative liberations of bio-agents are generally undertaken when natural enemies are absent; occur too late or in numbers too small to provide effective pest control. It was observed that after inoculative release of Zygogramma bicolorata in an area, colony gets started but it takes about 3-4 years for good control of Parthenium. Even in the sites where beetle has already established, it takes time to make sufficient population build-up, which is capable to suppress the Parthenium. In nature, good control of Parthenium by the beetle is observed by the end of September. This delay in population build-up may be reduced by augmentation of mass reared beetles coincided with the monsoon and germination of first flush of Parthenium in early June. It has also been observed that population build-up of beetles in the released area vary place-to-place and year-to-year. In some places, good control of Parthenium is observed even in the July and early August and in that area large number of adults and grubs of the beetles are available and from this population, large fraction of beetle and grubs succumb to death after complete defoliation. If, this population is transferred to less active site, good population build-up may occur which may lead to early control in the augmentative area.
For example, if we find good pupulaion build-up of grubs at one site, terminal twigs of Parthenium of 30 cm length may be cut and collected in buckets. These may be scattered over other Parthenium stand where population of beetle had not build-up yet. This inundative augmentation from the area of abundance to desire area will cause early population build-up in that area hence early control than the area where such augmentation is not made
The initial culture of bioagent Z. bicolorata can be obtained from the ICAR - Directorate of Weed Research (DWR) Jabalpur - 482004, Phone : 91-761-2353138.