Food is one of the major factors that regulate growth, reproduction and development in any insect species, including Z. bicolorata, commonly known as Mexican Beetle. Daily activities of Mexican Beetles are not only affected by abiotic factors, such as temperature, photoperiod and humidity, but are also influenced by quality and quantity of food they feed on. Consumption of food and its utilisation influence metabolism, enzyme synthesis, nutrient storage, and other activities that regulate growth and development of phytophagous chrysomelids. Thus, to understand the behaviour and ecology of these beetles, and to integrate them in pest management strategies as biocontrol agents, it is important to have a good knowledge of the ways in which these Mexican Beetles interact with their food sources.
P. hysterophorus L., commonly called Congress grass or carrot weed is an herbaceous, erect, annual plant (Family: Asteracae) native to the area around the Gulf of Mexico, including the West Indies and central South America (Rollins, 1950). The word ʻPartheniumʼ have been derived from Greek which means ʻfever fewʼ and refers to the use of Parthenin, a alkaloid present in the leaf and stem, as a remedy for fever and neuralgia; and the word ʻhysterophorusʼ is derived from Greek for ʻlater bearingʼ (McFadyen).
It has emerged as one of the worst weeds in several parts of Africa, Australia and Asia, including India. It is one of the most aggressive, obnoxious, (Oudhia, 2000) invasive weeds that has achieved wide distribution globally, affecting the growth of native species (King, 1974; Bhan et al., 1970). The weed shows a drastic impact on other plant as a strong competitor, and creates health hazards in humans and animals (Narasimhan et al., 1977; Rajkumar et al., 1988). The species interferes with the growth of other species by producing and releasing chemicals like phenolic acids, sesquiterpenes (Picman and Picman, 1984; Kohli and Batish, 1994) and other residues (Singh et al., 2003) which sieze the growth phenomenon of the coexistent species. Allelochemicals produced by the plant suppress growth of native vegetation posing a threat to biodiversity.
The weed was introduced in India in the early 1950s possibly from the USA through the imported wheat (Vertak, 1968) and has since spread to about 35 million hectare in 60 years (Sushilkumar and Varshney, 2007). Joshi (1991) describe this weed as a species with enormous biotic potential, facing very little environmental resistance, wastelands and on sides of railways, road easy and water channels, riverbanks where nitrogenous wastes of human and cattle are deposited regularly. Although primarily a wasteland weed, it has now moved into cropping areas in a very serious form. Parthenium posed a real threat in short stature crops and crop grown with wider spacing and reduced the forages production by 90 per cent. Wherever suitable moisture was available, Parthenium became the dominant species replacing all the beneficial crops.
To combat its menace, an integrated approach involved manually removing the plants (Joshi, 1990; Sharma and Gautam, 2004); use of herbicides like Paraquat, Atrazine, 2,4-D, Sodium salts etc.( Krishnamurthy et al., 1977; Muniyappa, 1980; Sharma and Gautam, 2005); cultural practices like intercultural operations and growing competitive crops like fodder, sorghum or maize (Joshi, 1990); biological methods using insects (Jayanth,1987; Jayanth and Bali, 1993) and by growing self perpetuating species like Cassia sericea (Joshi and Mahadevappa, 1986) that displaces Parthenium in course of time has been recommended. Manual and chemical methods are reported to be effective in controlling the weed to some extent in agricultural ecosystem. However, these methods are neither practical nor economical in pastures and wastelands as repeated applications are required. It also causes environmental pollution which is an allarming phenomenon now-a-days.
Under these situations, use of biological methods is considered to be sustainable and eco-friendly. The important insect pests that attack Parthenium in its native place are leaf feeding beetle, Zygogramma bicolorata, flower feeding weevil, Smicronys lutulentus, stem galling moth, Epiblema strenuana and fungal pathogens like Puccinia abrubta. Among these, Z. bicolorata reported to be effective in controlling the Parthenium in India, Australia, Nepal, South America and Vietnam.
The exotic insect, Z. bicolorata Pallister is an effective biocontrol agent of P. histerophorus (Jayanth, 1991; Dhileepan, Setter, and McFadyen, 2000; Sushilkumar, 2005; Dhileepan, 2007). The insect was imported from Mexico to Bangalore in 1983 and field releases were initiated in India in 1984 at Bangalore only. Now, the beetle became established in most parts of the South India and many parts of the Central and North India.
Mexican beetle, Z. bicolorata is a insect predator which have separate male and female sexes. Male measures 5.20 to 7.12 mm with an average of 6.67 mm and female 6.0 to 8.0 mm in length with an average of 7.36 mm. Female lays eggs singly or in group ranging from 2 to 80 eggs on leaves, flower, stem and buds of parthenium plant with an average of 40 eggs. Eggs are yellow in colour and hatch in 4 to 5 days. This variation in egg number laid per day may be due to climatic factors or due to single or multi time copulation or due to food quality.
There are four larval stages of creamy white colour and can easily be spotted on the plant. Larval duration extends from 3-5 days for 1st instars, 3-3.5 days for 2nd instars, 2-3 days for 3rd instars and 4-8 days for 4th instars. Total larval period lasts from 12 to 19.5 days with an average of 15 days. The larvae and adult beetles are voracious feeders and feed on leaves, tender stems and inflorescence, 4th instars larvae after growth, drops down on the soil near food plant root for pupation upto the depth of 10cm. Pupation period normally lasts for 10-12 days but it may be increased to 15-20 days in summer (June) and winter (November). This increased longevity may be attributed due to climatic change. Life cycle completed in 23-30 days with at least four generations depending upon humidity, temperature and quality of host plant (Dhileepan et al., 2000; Sushilkumar and Ray, 2011). It is a multivoltine species and generations cannot be differentiated under field conditions.