Deterioration of Grain Quality of Wheat by Rice Weevil, Sitophilus oryzae (L.) during Storage

Among various food grains, wheat is world-wide in its commercial cultivation on every continent except Antarctica. It had global production of 757.2 million tonnes (FAO, 2018) with about 22 per cent of production traded in international market. India is the second-largest wheat grower in the world. It was cultivated over an area of 29.1 and 3.5 million hectares with 102.2 and 17.8 million tons of production, respectively, in India (Anonymous, 2019a) and Punjab (Anonymous, 2019b). Several insect-pests like lesser grain borer, Rhyzopertha dominica (F.), rice weevil, Sitophilus oryzae (L.), granary weevil, Sitophilus granarius (L.) and Khapra beetle, Trogoderma granarium (Evert.) damage wheat in large or small quantities during storage. Of these insect-pests, S. oryzae is considered a primary pest of stored wheat (Burges, 2008; Mark et al. 2010). S. oryzae prefers soft textured wheat cultivars (Zaklanoi and Retanova, 1987). Ahmed (1980) reported 10-15 per cent losses due to various storage pests including S. oryzae in stored wheat. All the stored grain pests exhibit varied preference or non-preference for feeding over the grain samples of different wheat cultivars (Sarin and Sharma, 1983; Baloch and Irshad, 1986). Highest grain infestation in stored wheat by R. dominica and Khapra beetle, T. granarium was found responsible for the reduced crude fats and proteins while there was increase in crude fibres (Jood and Kapoor, 1993; Jood et al., 1993; Jha, 2003). Samuels and Modgil (2003) reported the negative correlation of crude fats and proteins with the insect pest infestation in different wheat varieties. However, Chuni and Singh (1996) revealed wheat cultivar HD-2307 as most resistant cultivar based on low population buildup of S. oryzae. The levels of different biochemical aspects like total proteins in stored grain samples vary with the incidence of a pest species concerned (Mason et al. 1997). The insect pests cause quantitative and qualitative losses to the grain samples during the storage (Fornal et al., 2007). Mebarkia et al., (2010) attributed the higher protein contents towards susceptibility of different wheat varieties to the S. granarius. Upadhyay and Srivastava (2010) found soft textured wheat cultivar PBW343 as the least preferred for insect infestation based on different biological aspects.

The present studies were carried out to investigate the deterioration of grain samples of different wheat cultivars by S. oryzae in the laboratory conditions.

The present investigations were carried out in the laboratory during year 2015 in the Department of Processing and Food Engineering, Punjab Agricultural University, Ludhiana. A pure culture of S. oryzae was raised on wheat cultivar PBW 2967 in the BOD at 27±1⁰C and RH 70±5 per cent. The insects were released in the plastic jars (size 12˝× 6˝), the open tops of which were covered with a piece of white muslin. A two hundred test insects were released into the plastic jars containing wheat grains for egg laying and removed after 24 hours. The new F₁ adults emerged in about 25-30 days were transferred to the new jars containing fresh food grains.

The adults from the pure insect culture were used for carrying out the present studies. The grain samples of six wheat cultivars, i.e., 3 bread (soft textured) viz., HD 2967, PBW 621, PBW 658 (soft texture) and 3 durum (hard textured) cultivars, viz., WHD 943, PDW 314, PDW 291 (hard texture) were used for carrying out the experiments. Each cultivar seed was properly cleaned and disinfested before its use. A 100g seed sample of each cultivar was taken into the plastic jars. From the pure culture, ten 1-2 week old young insects (1:1 sex ratio) were released into the jars each containing different cultivar grains. The jars were covered with white muslin as three separate storage sets, i.e., 30, 60 and 90 days after storage (DAS). Three replicates were kept in each treatment. One pre-storage observation was recorded for each storage interval. Observations were recorded based on the insect inflicted losses and biochemical losses.
 
A. Insect inflicted losses
 
Insect emergence

The insect population of S. oryzae was recorded from the 100 g food grain samples in all the six wheat cultivars at 30, 60 and 90 DAS.

Grain damage (%)

The grain samples of undamaged (without insect holes) and damaged (with insect holes) wheat samples were separated based on 1000 grain count at 30, 60 and 90 DAS to work out the grain damage.
 
Grain weight loss (%)

The grain weight loss was worked out as per the formula given by Adams (1976).

                         

                      
Wu = Weight of undamaged grains.
Nu = number of undamaged grains.
Wd = weight of damaged grains.
Nd = number of damaged grains.
 
B. Biochemical losses
 
A laboratory analysis of infested and insect free food grain samples was performed in all the wheat cultivars to determine ash contents, total proteins, fats and fibres at 30, 60 and 90 DAS (AOAC, 2000).
 
Statistical analysis
 
Experimental data was analyzed by Completely Randomized Design (factorial CRD) and square root transformations were computed for insect emergence by using computer programme CPCS1 (Cheema and Singh, 1990).

The S. oryzae showed less feeding response to wheat durum (hard textured cultivars) which had comparatively low proteins, total fats and fibre contents as compared to the soft textured cultivars (bread wheat). The ash contents and crude fibres showed positive while fats and proteins a negative correlation with pest infestation in wheat cultivars (Table 1-3).




 


At 30 DAS, the food grain samples in infested cultivar HD 2967 registered highest insect emergence (8.67 adults), grain damage (2.93%), weight loss (0.33%), ashes (2.49%) and fibres (4.67%) while decline in total fats (1.54%) and proteins (13.65%) over the insect free samples having declined ashes (2.40%), fibres (4.60%) and increased fats (1.65%) and proteins (13.70%). In contrast, the infested grain samples of wheat variety PDW 291 registered a significant reduction of insect inflicted aspects like insect emergence (2.33 adults), grain damage (0.67%), weight loss (0.03%) and biochemical properties such as crude fats (0.83%) and proteins (10.71%) while increase in ashes (1.99%) and fibres (3.06%) over the uninfested grain  samples having reduced ashes (1.85%) and fibres (2.90%) while rise in fats (0.86%) and proteins (10.79%).

At 60 DAS, food grain samples of infested cultivar HD 2967 revealed increased insect emergence (60.00 adults), grain damage (12.90%), weight loss (1.50%), ashes (2.61%), fibres (4.81%) and declined total fats (1.52%) and proteins (13.58 %) as against insect free samples with ashes (2.40%), fibres (4.61%), fats (1.64%) and proteins (13.71%). However, infested cultivar PDW 291 showed a declined insect emergence (14.33 adults), grain damage (2.20%), weight loss (0.10%), fats (0.79%) and proteins (10.70%) while increased ashes (2.07%) and fibres (3.37%) over the insect free cultivar with declined ashes (1.86%), fibres (2.93%) and increased fats (0.85%) and proteins (12.43%).

Similarly, at 90 DAS, food grain samples in infested cultivar HD 2967 showed increased insect emergence (135.00 adults), grain damage (26.63%), weight loss (9.66%), ashes (2.65 %), fibres (5.01%) and declined fats (1.35%) and proteins (13.53%) as compared to insect free samples with ashes (2.41%) and fibres (4.62%), fats (1.65%) and proteins (13.67%). In contrast, the infested cultivar PDW 291 showed declined insect emergence (30.67 adults), grain damage (4.27%), weight loss (1.97%), fats (0.76%), proteins (10.38%) and increased ashes (2.25%) and fibres (3.59%) as against insect free cultivars with declined ashes (1.84%) and fibres (2.94%) and increased fats (0.86%) and proteins (10.77%).

A highest and lowest grain damage (14.15 and 2.38%) and weight loss (3.83 and 0.70%) due to S. oryzae in cultivars HD 2967 and PDW 291, respectively, was as per Dwivedi and Shukla (2019) who also recorded lesser grain weight loss and grain damage in wheat varieties HD-2733 and K-307. A direct correlation in total ashes with pest infestation was supported by Hameed et al., (1984) for T. granarium on wheat. A high insect emergence in soft textured cultivars- HD 2967 (84.67 adults), PBW 621 (62.44 adults), PBW 658 (56.00 adults) and low in the hard ones- WHD 943 (48.56 adults), PDW 314 (34.78 adults) and PDW 291 (25.44 adults), was confirmed by Paudel et al., (2003) in T. granarium resistant and susceptible maize cultivars. The current studies for declined proteins and total fats due increased S. oryzae infestation were confirmed by Mahmood et al., (2013) for acarid mite, Rhizoglyphus tritici. The increased fibres and ashes in insect free and infested food grain samples in their findings also supported the present research work. A highest insect emergence, grain damage and weight loss in R. dominica infested grain samples in cultivar HD2967 in current findings was fully supported by Kakde et al., (2014) for wheat cultivar HD2329. A highest susceptibility based on insect activity and biochemical aspects in bread wheat cultivar HD2967 due to pest infestation in present findings was supported by Arve et al., (2014) for bread wheat cultivar HD2189. A negative correlation of S. oryzae infestation with total fats and proteins while positive with ashes and fibres in present studies was partially supported by Keskin and Ozkaya (2015) who reported a negative correlation of S. granarius infestation with total fats but positive correlation with proteins and ashes.

The grain samples infested cultivar HD 2967 encountered an increase in insect emergence, grain damage, weight loss, total ashes, crude fibres while decrease in total fats and proteins over the insect free cultivars. In contrast, infested grain samples of cultivar PDW 291 showed decline in insect emergence, grain damage, weight loss, fats, proteins but rise in total ashes and fibres over the insect free food grain samples.

All authors declare that they have no conflict of interest.