Abstract:
This work demonstrates how an integrated approach can be used for controlling Larger Grain Borer (Prostephanus truncatus) in maize (Zea mays L.) using solar disinfestation followed by hermetic storage in Purdue Improved Crop Storage (PICS) bags. PICS bags work by sealing grains in an airtight environment for long-term pest-free storage. In order to provide safe food with no inputs of chemical pesticides, the combination of solar disinfestation and the PICS bagging is likely to be a promising integrated approach for controlling P. truncatus. The first experiment of this work was the heat disinfestation experiment. The aim of this experiment was to come up with a simple way of reducing P. truncatus populations as an adjunct to hermetic storage of maize grains in PICS (Purdue Improved Crop Storage) bags. The oven tests were supposed to lay the basis for large scale disinfestation of maize grains using solar energy. As a result it would eventually be determined if solar disinfestation and hermetic storage can be combined to form an effective hurdle technology to prevent postharvest losses of maize due to insect damage especially P. truncatus. As such, this describes an integrated approach towards the management of P. truncatus. Maize infested with adult insects of Prostephanus truncatus was used to study the heat tolerance of P. truncatus so as to lay the basis for solar disinfestation of maize prior to storage. The experiments were carried out in a Memmert (B54 Scwabach, Western Germany) air oven for different time-temperature combinations. The time-mortality data was first subjected to analysis of covariance and variance (ANCOVA) to determine the effect of time, temperature and the interaction of time and temperature on the mortality of P. truncatus. Time had no significant effect (p > 0.05) and as a result one-way ANOVA was done for each exposure time separately. The oven tests showed a critical temperature of 60˚C as essential to effect a near complete kill of 98.5 % mortality for an exposure period of 90 min. Exposure to higher temperatures of 65 and 70˚C of 60 min and 30 min was able to achieve 98 % and 100 % mortality respectively. Complementary log-log transformation (logistic regression) analysis was done as well and the lethal dose values were calculated to determine differences in lethal temperature (LT) values among the four exposure times (30, 60, 90 and 120 min). The LT95 for an exposure time of 120 min was 55.3˚C while for 60 min it was 61.3˚C. Longer exposure times resulted in further significant reduction in lethal temperature values. Preliminary solar disinfestation experiments showed that it was possible to achieve up to 60˚C on a sunny day with temperatures of at least 26˚C between 11:30 a.m. and 2:30 p.m. when the sun is hottest. Therefore, it was concluded that heat disinfestation is an effective low-cost alternative to maize grain fumigation to control P. truncatus in Sub-Saharan African. The second experiment aimed at comparing the performance of hermetic PICS bags to woven polypropylene bags in the storage of maize for 6 months. It has been established that hermetic post-harvest maize storage can effectively control maize weevil, Sitophilus zeamais, which can be responsible for up to 50 % damage to stored maize grain. Its use eliminates the need for toxic and expensive chemicals. Therefore, laboratory experiments with Prostephanus truncatus Horn which causes more severe damage were carried out. Maize infested as well as non-infested with P. truncatus, was stored for 6 months under hermetic (PICS bags) and non-hermetic (woven polypropylene (PP bags) conditions. Grain moisture content (m.c.) at the beginning of the experiment in January 2012 ranged from 12.30 to 13.31 % and the weight loss damage was 0%. Under hermetic conditions, after six months‘ storage gas composition levels were at 6.82, 7.68 and 9.34% and CO2 level had risen to 13.52, 12.75 and 9.88 %. Insect counts were low in the PICS bags, 2 ± 1 insects per 125 g sample; and very high in the woven bags, 52.00 ± 9.85 live P.truncatus, 68.67±9.07 P. truncatus larvae and 74.33±12.10 S. zeamais per 125 g sample in the worst case. Germination capacity did not change much for the PICS bags but decreased greatly for woven bags up to 12% in the worst case. Losses were also significantly different for the maize stored in the PICS bags and the woven bags with losses as high as 47.66±4.59 % recorded for the woven bags. In terms of effectiveness in storing maize without damage, after six months of storage, PICS bags appeared to be better compared to PP bags. As there is no difference between the PICS bag with maize that was artificially infested with P. truncatus then solar disinfested (T2) and PICS bag with maize that was solar disinfested then artificially infested with P. truncatus prior to bagging (T3), it is likely that PICS bags do not need any integration with solar disinfestation before performing. More studies are needed to confirm the results of the present study. Hermetic storage is an effective low cost-effective system for grain produced in the rural areas of developing countries.Keywords: Maize (Zea mays L.), Prostephanus truncatus, heat disinfestation, hermetic storage, Purdue Improved Crop Storage (PICS) bagging.