Assessment of approaches to obtain ebullition pressures for organophilic clay blankets

The objective of this study is to compare two experimental approaches to characterize the ebullition pressure (or air-entry suction) of initially water-saturated organophilic clay blankets. The first is an indirect approach using the water-retention curve (WRC) and the second is a direct approach using ebullition experiments. The WRC along with the hydraulic conductivity of organophilic clay blankets in saturated and unsaturated conditions were measured using a flexible-wall permeameter with suction-saturation control. This device was also adapted to measure the ebullition pressure and the air permeability. The comparison of the experimental approaches was performed on organophilic clay blanket specimens in different initial conditions (unrinsed and rinsed to remove loose fines) under high and low effective confining stresses (20 and 5 kPa). The indirect estimates of air-entry suction from the WRC were similar to those obtained from the ebullition tests. This good agreement between the two approaches may add flexibility to the development of design specifications for capping systems. The hydraulic properties were found to be sensitive to rinsing and effective stress, with greater hydraulic conductivity and air permeability for the rinsed specimen due to the removal of fines, and greater air-entry suctions for specimens under higher effective stress

Aphid populations (Hemiptera: Aphidoidea) depend of mulching in watermelon production in the Mediterranean region of Croatia

Winged morphs of aphids were investigated under field conditions during 2008, 2010 and 2011 in the Mediterranean region of Croatia. Field experiments were conducted to record aphid diversity and compare polyethylene black mulch to straw mulch and bare soil in terms of their attractiveness to aphid species in a watermelon crop. Aphids were collected weekly using yellow water metal traps from May to July. During the study, 44 species in 31 genera were detected; 36 species in 24 genera were identified in 2008, 18 species in 15 genera were identified in 2010, and 34 species in 25 genera were identified in 2011. The overall seasonal percentage composition showed that Aphis fabae Scopoli and Myzus persicae (Sulzer) were consistently eudominant species, whereas Acyrtosiphon pisum (Harris) was consistently dominant. Cabbage aphid Brevicoryne brassicae (L.) was determined as the dominant species in 2008 and as eudominant species in 2011. Additionally, Macrosiphum rosae (L.) was twice recorded as the dominant species, whereas Aphis gossipii Glover was dominant once, and Phorodon humuli (Schrank) was eudominant once. Our study also demonstrated that mulching sporadically affected the abundance of individual species. The attractiveness of mulching for aphid species differed between the treatments. It was found that bare soil was attractive to A. pisum and M. rosae, black mulch was attractive to B. helichrysi and P. humuli, and straw mulch was attractive to four aphid species, of which two were eudominant, A. fabae and M. persicae. The presented species are mostly polyphagous, and their preference for certain mulches can be used in crop protection management

Correlation between epicuticular wax content in the leaves of early white cabbage (Brassica oleracea L. var. capitata) and damage caused by Thrips tabaci Lindeman (Thysanoptera: Thripidae)

In the period of 2001-2002 the susceptibility of five varieties of early white cabbage to onion thrips (Thrips tabaci) attack has been studied. The variety with the longest growth period (72 days) and the firmest head (19.5-21.5 kg/cm2), i.e. Vestri, had the highest mean weight of the head (1220.0 g/head) and the highest net weight of the head (1050.6 g/head), while the mean index of damage (1.18) and the mean yield loss (13.9%) were the lowest in this variety. At higher plant density (16.6 plants/m2) both parameters of the weight of the head were approximately less than half as compared to lower plant density (8.2 plants/m2). Results of the mean net weight of the heads did not show significant differences between different plant densities if the yield was calculated per area unit (m2). The mean content of epicuticular wax differs among varieties, although all varieties show similar relations in its content; the lowest mean content in the third and the fourth exterior leaves of the head, the highest mean content was within the first and the second, and the ninth and the tenth exterior leaves of the head. In spite of the apparent correlation between the content of epicuticular wax and the mean index of damage (with both plant densities its values were the highest between the third and the sixth exterior leaf of the head) caused by feeding of onion thrips hardly perceived negative correlation (Pearson coefficient of correlation between -0.338 and -0.436) were found between these two parameters using statistical analysis