Parthenium weed is an invasive species in a growing number of countries where it infests numerous crop fields, including sorghum. Two field studies were conducted to quantify the effect of parthenium weed on the performance of grain sorghum at different weed densities (0, 5, 10, 15 and 20 plants m−2) and durations of weed-crop competition (season-long weed-free, weed-free after 2, 4, 6 or 8 weeks, and season-long weedy). Our aim was to identify the density threshold and ideal duration to control parthenium weed in sorghum fields. Both field experiments were planned in a randomised complete block design each with three replications in 2016 and were repeated in 2017. Parthenium weed biomass increased significantly with increasing density and competition duration. The increasing parthenium weed density had a linear negative effect on sorghum growth, yield and yield-contributing traits. The highest yield loss, of up to 66%, was recorded at the highest parthenium weed density of 20 plants m−2 when compared to weed-free treatment. In addition, the season-long competition of this weed with sorghum caused 81% reduction in grain yield over weed-free treatment. According to our results, parthenium weed should be managed below a density of 5 plants m−2 and throughout the crop growth duration in grain sorghum fields as it can cause serious yield losses even at low densities and through strong competition at early as well as late growth stages of the crop.

Zygogramma bicolorata Pallister is a potential biocontrol agent of Parthenium hysterophorus L. (Asteraceae). This study was carried out to understand the effect of abiotic (temperature and moisture) and biotic factors (age and reproductive status) on the induction and termination of diapause in Z. bicolorata. We also evaluated the effect of diapause on longevity and fecundity of Z. bicolorata. In Z. bicolorata the induction of diapause occurred between the months of July and December. No diapause was observed in May and June, which coincided with the commencement of the monsoon rains. The percentage of diapause induction in Z. bicolorata was found to increase from one generation to another as well as with the age of adults. Diapause significantly increased the fecundity of females as compared to the fecundity in pre-diapause conditions. Soil moisture (80% RH) played an important role in providing the conditions for initiation and termination of diapause. Similarly, 18.4 and 12.5% of diapause adults resumed their activity in 6 and 5 days when exposed to temperatures of 40° and 45° C for 2 h every day, respectively. Exposure of adults to low temperatures, i.e. 5° and 10° C induced 94.3 and 92.5% diapause, respectively, with no adult mortality. Thus, our findings reveal suitable conditions for preventing and inducing diapause in Z. bicolorata, which is of great importance in the suppression of P. hysterophorus. We discuss the implications of these findings in the control of P. hysterophorus.

The demographic attributes of the parthenium beetle Zygogramma bicolorata Pallister were studied using Parthenium hysterophorus L. as food under different abiotic and biotic variables. Intrinsic and finite rates of increase were highest with the shortest mean generation and doubling times when Z. bicolorata was fed on inflorescences followed by the leaves and stems of P. hysterophorus. Temperature significantly affected the demographic parameters. The net reproductive rate was highest at 27 °C and lowest at 35 °C. Different photoperiodic exposures altered the demographic attributes of the beetles with long-day exposure (14L:10D) resulting in optimal values compared with other photoperiodic exposures. Continuous darkness (0L:24D) and continuous light (24L:0D) suboptimally affected the demographic attributes of the beetles. In response to the light of different wavelengths, the net reproductive rate, and intrinsic and finite rates of increase were significantly higher when the beetles were reared under a broad spectrum of white light followed by those under ~570 nm (yellow), ~475 nm (blue) and ~650 nm (red). Information gained in this study may be helpful in the mass multiplication of Z. bicolorata, and thereby strengthening the biocontrol of parthenium weed.

The effect of different constant temperatures on several reproductive attributes of the Mexican beetle, Zygogramma bicolorata Pallister, was studied. Fecundity and egg viability, pre- and post-oviposition period, and age-specific fecundity were assessed at 20, 25, 27, 30 and 35 °C. Maximum fecundity and per cent egg viability were recorded at 27 °C (928 eggs and 75.6%, respectively). Pre-oviposition and post-oviposition period declined with increasing temperature up to 27 °C, then increased at temperatures beyond that. Oviposition period decreased from 92.9 to 27.5 days with temperature increment. Age-specific fecundity was also temperature dependent. Z. bicolorata is a biological control agent of the parthenium weed Parthenium hysterophorus Linnaeus, and these findings may be useful in mass rearing programmes for the natural enemy.

The effect of different constant temperatures (20, 25, 27, 30 and 35 °C) on life history stages of the Mexican chrysomelid beetle, Zygogramma bicolorata Pallister, fed on the weed Parthenium hysterophorus Linnaeus was investigated to analyse the development and survival of different life stages and the adult sex ratio. Fastest development and maximum survival were recorded at 27 °C. The first instar was the most vulnerable, while the pupa was least vulnerable. For complete development, the lower temperature threshold (t°C) was 18.5 °C and the thermal constant (K) was 480.8 day-degrees. A higher percentage of females was obtained at all temperatures; however, this was unaffected by the temperature regime. This study established that the percentage life span spent by immature stages was similar across all five temperatures, indicating developmental rate isomorphy. 27 °C was the most suitable temperature under controlled abiotic conditions for the propagation of Z. bicolorata, a bioagent for parthenium weed.

To understand germination timing in an ecological context, the response to environmental events that affect seed dormancy is central, and has to be combined with knowledge of germination responses to different circumstances. In this study, seed dormancy, germination and emergence phenology of six annual co-occurring weedy Asteraceae species were investigated. Three pre-treatments (warm and cold stratification, and dry storage) were tested as possible dormancy-affecting environmental events. Seeds were also sown outdoors. Species-specific differences were revealed in analyses. To facilitate general descriptions of dormancy patterns and germination preferences separately, condensed responses to the different possible dormancy-affecting treatments and relative germination in different environments were plotted, giving species-specific patterns. Most species exhibited decreased dormancy to two or three pre-treatments. Dormancy was most effectively reduced by cold stratification for three species (Guizotia scabra, Parthenium hysterophorus, Verbesina encelioides), by warm stratification for two (Bidens pilosa, Galinsoga parviflora) and by dry storage for one (Tagetes minuta). All species germinated more when provided with light than in continuous darkness. Temperature levels most suitable for germination varied from low (15/5–20/10°C) for V. encelioides to high (25/15–30/20°C) for B. pilosa. It is concluded that, even though the species have different dormancy patterns and germination preferences that suggest different possible distribution ranges, they achieve similar emergence timing in the field in environments with a pronounced dry period after dispersal and small annual temperature fluctuations.

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.

Biological control efforts on Parthenium hysterophorus L. (Asteraceae) in India have gained momentum after the limitations of other methods were realized. Native surveys revealed a large number of insects, but none of them was host specific. Although the introduced beetle Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae) has established at the sites of initial releases, its real impact on the weed and performance in different parts of the country need further evaluation. Fungal pathogens of the weed hold promise for classical as well as microherbicidal control. The use of parthenium phyllody MLO as a biocontrol agent requires establishment of host and vector specificity. Mycotoxins are a potential group of herbicides on which serious studies are yet to begin. Studies on control of the weed through interference and allelopathy by Cassia uniflora Mill. (= C. sericea Sw.) (Leguminosae) have produced promising results. Toxic leachates of C. uniflora and autotoxic principles of the weed deserve attention. Integrated biocontrol strategies envisaged for wastelands using introduced insects and pathogens, allelopathic plants, and agroecosystems using native pathogens, mycotoxins and autotoxic principles, will help combat this apparently invincible weed.