Search

Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae), a biological control agent of Parthenium hysterophorus L. (Asteraceae), was feeding on leaves of Helianthus annuus in India, raising concerns of its host range. Based on the age grading technique, it was shown that the majority of Z. bicolorata on sunflower plants at any time were reproductively immature. This establishes that Z. bicolorata does not pose any risk to sunflower in India.

Freshly emerged adults of Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae), introduced for biological control trials against the noxious weed Parthenium hysterophorus , were seen to feed and lay eggs on sunflower leaves under confined conditions in the laboratory. The time taken for initiation of feeding was found to be positively correlated with adult age, while survival after feeding on the crop was negatively correlated. However, adults that had fed on sunflower even for 1 day on P. hysterophorus were incapable of ovipositing, and young larvae could not feed on the crop. Under field conditions, newly emerged adults were only attracted to feed on plants that were treated either with a crude aqueous extract of leaves or pollen grains of P. hysterophorus , indicating that the chances of Z. bicolorata becoming a pest of sunflower are negligible. Age-related Feeding by the Parthenium Beetle Zygogramma bicolorata on Sunflower and its EVect on Survival and Reproduction

Observations under field conditions in Bangalore showed that release of Neochetina eichhorniae and N. bruchi (Coleoptera: Curculionidae) brought about successful control of the water hyacinth in water bodies where the plants were free-floating. However, in situations where the plants were partly anchored, suppression of the weed was achieved only 8 years after insect release. Studies carried out under glasshouse conditions, in which plants with free-floating and silt-covered roots were exposed to Neochetina spp., showed that fully grown larvae were incapable of pupating on silted roots of the water hyacinth. This resulted in reduced adult emergence of N. eichhorniae and N. bruchi, indicating that delayed suppression of water hyacinth in silted tanks may be due to the low population build-up of the weevils.

Time-specific life tables were constructed for the Mexican beetle, Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae), which is a promising biological control agent of the Neotropical weed, Parthenium hysterophorus L. (Asteraceae) in and around Bangalore, India. Infertility of eggs (46.00–71.10%) was found to be the key mortality factor, followed by death during the pupal stage (15.76–21.38%), as confirmed by key mortality factor analysis and construction of survivorship curves. No parasitoids or predators were recorded on this insect, while some of the dead pupae/pre-pupae (2.27–17.24%) were found to be infected by Metarhizium anisopliae (Metchnikoff) Sorokin. The high generation trend values (305.88–557.14), give an indication of the biotic potential of this insect, which was found to cause rapid defoliation of the weed under field conditions.

Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae) of Mexican origin, introduced for biological control trials against the weed Parthenium hysterophorus (Compositae), was found to feed on the tender leaves of the related plant sunflower (Helianthus annuus Linnaeus) at a few places in Kolar district in Karnataka State, India. The insects in these populations exhibited variability in feeding response to sunflower under laboratory and field conditions. The sesqueterpene lactone parthenin, specific to P. hysterophorus, was found to induce feeding by the adults of Z. bicolorata on treated sunflower leaves within six hours, as compared to 4–7 days on untreated leaves. Further investigations resulted in the novel finding that dusting of parthenium pollen, or smearing its extract which contains parthenin, on sunflower leaves also induced feeding responses in adults. Parthenium pollen was detected on sunflower leaves growing in close proximity to the weed stand, suggesting that this could be one of the factors influencing variability in the feeding behaviour of Z. bicolorata.

Zygogramma bicolorata Pallister was introduced for biological control trials against the weed Parthenium hysterophorus (Asteraceae) in India. The insect entered diapause over an extended period of time between July and December in Bangalore. Diapausing adults burrowed into the soil, and emerged in May–June with the commencement of monsoon rains. Percentage diapause increased over time, peaking at 72% during November. Non-diapausing adults were capable of breeding, under laboratory conditions, during the winter. Some adults bred both before and after diapause, during two consecutive years. Soil moisture played an important role in providing the conditions for burrowing or emerging from the diapause chambers. The studies also showed that diapausing adults had to be exposed to the high summer temperatures, for termination of the diapause. It was possible to break diapause by continuous exposure to 30°C, 35°C and 40°C for 22 days, nine days and 10 hours, respectively, during February–March, about three months after its inception. This method can be used to initiate mass multiplication for carrying out releases early in the season.

Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae) of Mexican origin, released in Bangalore in 1984, was observed defoliating Parthenium hysterophorus L. (Asteraceae) in 1988, and had dispersed over 50 000 km2 by 1992. The insect appears to have potential in reducing weed density in those parts of India having moderate weather conditions.

Weeds play an important role in human affairs over much of the earth. The major characteristics of weeds are their unwanted occurrence, undesirable features and ability to adopt to a disturbed environment (Combellack, 1992). Despite measures adopted for their control, weeds are estimated to reduce world food supplies by about 11.5% annually ((Combellack, 1992Parker and Fryer, 1975). Many of our problem weeds are of exotic origin, having been introduced accidentally or deliberately as ornamental plants, etc. They flourish in the new environment as they have escaped from the natural enemies, which suppress their vigour and aggressiveness in their native lands.

Water hyacinth is the most serious aquatic weed in India, infesting more than 200000 ha of water surface. Successful biological control of this weed was achieved in a fully infested tank covering 20 ha at Bangalore. More than 95% of the infestation was cleared within 32 months by releases of Neochetina eichhorniae Warner. The insect also spread along a water channel and controlled water hyacinth infesting a nearby tank. Drying up of the tank after the insect population had spread throughout and increased to 3.1 adults/plant hastened weed control. Fresh plants emerging after rains were suppressed by the residual population of insects. N. eichhorniae along with N. bruchi Hustache is being supplied to centres throughout the country for multiplication and releases. Sustained efforts with these insects holds out promise of control of water hyacinth in India.

Melanaspis glomerate (Green) is a serious pest of sugarcane in India and does not have effective natural enemies. Parasites of M. smilacis Comst. and M. paulislus Hempel from Brazil and Aphytis mytilaspidis (Le Baron) collected from Aulacaspis tegalensis (Zehnt.) in Mauritius were imported for biological control trials. Only Neococcidencyrtus sp., an endoparasite of M. paulistus accepted M. glomerata. Among the coccinellid predators that were imported only Chilocorus cacti (L.) bred on M. glomerata. The non‐acceptance of this scale insect by most of the introduced natural enemies seems to indicate the need for confirmatory studies on its taxonomic status. Until such time as this is done and suitable natural enemies are found, an integrated approach for its control may have to be considered.

The aquatic fernSalvinia molesta D.S. Mitchell is a serious weed in India, espacially in the state of Kerala. The weevilCyrtobagous salviniae Calder and Sands (Col.: Curculionidae) was introduced from Austrialia and successfully controlled the weed infesting a lily pond at Bangalore.