Efficient solar heater box for control of Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) on cocoa beans

Insect infestation is one of the factors that is normally considered during the grading of cocoa beans. Tribolium castaneum (Herbst) is one of the insect pests that has been consistently found attacking cocoa beans in Malaysia. Solar heater boxes of smaller sizes have been found to be effective in collection and retention of solar radiation as heat at levels lethal to stored product arthropod pest inside the box. The need to determine the damage potential of T. castaneum on cocoa beans and to develop solar heater box that could handle large amount of commodity lead to this study with the following objectives: (1 ) to determine the damage potential of T. castaneum on Malaysia cocoa beans (2) to construct a solar heater boxes of different sizes and assess their solar heat-trapping capacity as well as the effect of quantity of cocoa beans and exposure time on the heat trapping ability of the solar heater box (3) to evaluate the effect of solar heat trapped by best performing solar heater box on the life stages of T. castaneum (4) to examine the effect of solar heat treatment at different exposure times in cardboard solar heater box on the quality attributes of cocoa beans. Damage potentials of T. castaneum on cocoa beans were investigated under laboratory conditions by focusing on the effect of different initial population densities (0, 10, 20 and 30) at five post infestation storage times (30, 60, 90, 120 and 150 days) as well as their interactions on final adult population density, percentage of insect damaged cocoa beans, percentage of weight loss, and weight of insect feeding residues. Both factors and the associated interaction had a highly significant (P<0.01) effects on all the response variables assessed. The highest mean final population density of 129.67±4.60 was recorded in samples infested with 30 adults and stored for 150 d, while the least mean adult population density of 10.81±0.54 were recorded on samples infested with 10 adults and stored for 30 days. The highest percentage (50.97±1.21%) insect damaged cocoa beans were recorded in samples infested with 30 individuals, while the least mean percentage (16.88±1.26%) insect damaged cocoa beans were also recorded on samples infested with the lowest 10 adults and stored for 30 days. Similar trends of means were recorded for all the remaining other response variables. Correlations between variables were significant and positive. Multiple and simple linear regressions analyses were significant (P<0.01) and all equations fitted the regression lines perfectly and hence adequately described the relationship between independent and dependent variables. On the overall, the result shows that T. castaneum can impact negatively on both the quantity and quality of stored cocoa within just 30 days of infestation, with the impact increasing with increasing population density and post infestation storage time. The effect of sizes of solar heater boxes on their heat-trapping efficiency was studied by exposing solar heater boxes of five different sizes to solar radiation for 5 h. The influence of the quantity of cocoa beans and length of exposure time on heat collection capabilities of the best performing solar heater box was evaluated using five different quantities of cocoa beans (9, 12, 15, 18 and 21 kg) for 30,60 90 and 120 min of an exposure period. The result for the effect of size on heat-trapping capacity shows that the size of solar heater box had a significant effect on the amount of heat trapped in the solar heater boxes. The largest solar heater box trapped the highest mean between and within cocoa beans temperatures (69.38±4.97 and 69.45±3.97ᵒC respectively) over the 5h of exposure time. All bigger heater boxes trapped significantly higher mean temperatures that of the control solar heater box. Heat trapped in solar heater box were significantly affected by the quantity of cocoa beans and length of exposure time. Quantity was inversely related to trapped heat, while, exposure time was proportionally related to trapped heat. Interaction effects were also significant. The highest mean temperatures were obtained at 120 min exposure time using 9kg of cocoa beans for both between and within bean temperature (70.00±0.73 and 71.23±0.850C respectively), while the least mean temperatures between cocoa bean (43.03±1.920C) and within beans temperature (44.83±3.090C) were obtained when 21 kg of cocoa beans were exposed for 30 min. The effects of heat treatment in solar heater box on life stages of T. castaneum infested on cocoa beans were examined by artificially infesting 30 adults, larvae, eggs and pupae of T. castaneum on one (1) kg of cocoa beans in fabric bags and exposure to solar radiation in solar heater box for 30, 45, 60, 90 and 120 min. Adult and larval mortalities were assessed 24 h after exposure until 72 h. Eggs hatchability and pupal eclosions were also assessed daily for 9 and 7 days respectively after exposure. The results show that the solar heat treatment had a significant lethal effect on all the life stages. Therefore, regardless of exposure time, absolute mortalities were recorded in both the adult and larval population treated in the solar heater box. Similarly, both egg hatchability and adult emergence from pupal stages were effectively inhibited in a similar manner for all solar heater box treated groups. The effect of heat treatment in solar heater box on the proximate and mineral composition, pH levels of the sample of cocoa beans and free fatty acid content of the fats extracted from the treated cocoa beans were determined using standard procedures and protocol for same in cocoa beans. The result shows that solar heat treatment at temperatures lethal to insect pest did not affect any of the quality characteristics of the cocoa beans over the tested periods. On the overall, it was concluded that T. castaneum can cause substantial damage on infested cocoa beans. The bigger the solar heater box, the higher the trapped temperature and quantity of beans was inversely related to temperature. All treated life stages of T. castaneum suffered absolute mortality with no effect on cocoa beans quality parameters. Hence, the cardboard solar heater box used for this study is an efficient and reliable tool for heat sequestrations for the eco-friendly control of T. castaneum on cocoa beans.

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