The experiment was conducted in the Crop Protection Laboratory of the Department of Crop and Soil Science, Faculty of Agriculture, University of Port Harcourt, Nigeria. The temperature and relative humidity of the experimental site were 25 ± 2 °C and 75 ± 5%, respectively, during the period of the study. The site had sufficient oxygen penetration (windows were left open) for the optimum performance of the rice weevil, Sitophilus oryzae L. insect.
Ten different brands of rice which are commercially available were used for the study. These were royale stallion, Mama royale, parboiled rice, Mama gold, white rice, Super eagle, Indian rice, champion rice, Abakiliki rice and Mama Africa. They were purchased at random from rice stores in Port Harcourt city, Rivers State, Nigeria. Four kilograms of each sample was put in a small black polythene bag and carefully tied and labeled with the aid of a string. The samples were transported to the laboratory for palatability and appearance tests which were carried out immediately.
Palatability and appearance tests
Two kilograms of each of the ten rice brands was cooked separately with water only, for 30 min on a Binatone gas cooker until soft to determine palatability. To rate the brands according to palatability, 50 panelists (drawn from University of Port Harcourt, Nigeria) were randomly selected and served with each of the cooked rice brands coded with a letter to reduce prejudice. Each panelist ate a spoon full of each rice brand and rinsed mouth with water before tasting the other brand to prevent the cross-palatability effect. The panelists rated the palatability of the rice brands using a scale of 1 to 5 scores in a form provided. The scores of 1 to 5 corresponded to poor, fair, fairly good, good and excellent. Twenty grams of each rice brand (also coded) was presented alongside cooked samples for visual examination by the panelists who also scored their observations as described above.
Local rice infested by rice weevil (S. oryzae) was obtained from the open market in Choba, Rivers State, Nigeria, and kept in five separate 1-L Kilner jars in the laboratory where weevil identity was confirmed by an expert. The rice weevils were sieved out using a plastic laboratory test sieve and transferred to five separate 1-L Kilner jars containing fresh Abakiliki rice. After 7 days, when the weevils have fed and oviposited, they were sieved out. The setup was allowed to stay for 35 days until emergence of new progenies began. New adult progenies of similar age (7 days old) were used for the experiment.
For susceptibility test, only fresh intact rice grains were used. Damaged grains and other irrelevant materials were sorted out and discarded. Thirty grams of all rice brands weighed using the sensitive electronic Mettler balance (model mp 2003) was disinfested with cold temperature by keeping them in a refrigerator for 7 days, and the grains were considered completely disinfested (Khan and Halder 2012). The disinfested grains were spread on a muslin cloth and allowed to acclimatize at room temperature and relative humidity for 72 h to bring them to normal condition. The average moisture content of the rice samples (11.5%) was determined using the standard oven method (Khan and Halder 2012). Twenty grams of standardized grains of each rice brand was introduced separately into a 100-ml capacity jar. Two pairs (2♀ + 2♂) of 7-day-old S. oryzae sexed morphologically as described by Halstead (1963) were released into each jar and covered with muslin net, arranged in a completely randomized design on the workbench and replicated four times. The parent weevils were left for 7 days to feed and lay eggs, after which they (living and dead) were sieved out. The containers were kept undisturbed under the condition of normal room temperature and relative humidity (25 ± 2 °C and 75 ± 5%). Insect count per brand began 35 days post-infestation when the F1 progenies have started emerging up to 56 days post-infestation duration. The weight (g) of F1 male and female progenies of S. oryzae was also recorded per rice brand. The seeds in each container were measured to determine percent weight loss on the 57th day post-infestation. The weight of frass (g) was also measured at the same day.
Data of number of emerged progenies, weight of emerged progenies and weight of frass were analyzed using one-way analysis of variance, and significantly different means were separated using Tukey’s studentized range (HSD) at α = 5%. The software was Statistical Package for the Social Science (SPSS), version 19.0. Palatability and appearance were scored, described and presented in a table, while associations of rice brand palatability and appearance with susceptibility indices were assessed using electronic scatter plots.