Evaluation of plastic packaging for prevention of damage to wheat by Trogoderma granarium (Coleoptera: Dermestidae), and suitability of phosphine fumigation

We evaluated 4 types of plastic packaging containing whole wheat grains for damage to packaging in the form of scratches (damage through which insects cannot enter packages), holes (damage through which insects can enter packages), and penetrations (insects entering the packaging through holes) by Trogoderma granarium (Everts) (Coleoptera: Dermestidae) during the third and fourth instars. The plastic packaging used in this study was opaque polyethylene (high density), polypropylene, transparent polyethylene (low density), and polyvinylchloride with thicknesses of 0.021, 0.023, 0.026, and 0.036 mm, respectively. After 90 d, damage (scratches) was common with polypropylene and transparent polyethylene packaging, less so with opaque polyethylene, and no damage was found with polyvinylchloride packaging. Holes were more frequent in polypropylene, followed by opaque polyethylene, but none in the other forms of packaging. Insect penetrations were more frequent in opaque polyethylene, followed by polypropylene, but none in other forms of packaging. We also determined the number of damaged grains, weight of damaged grains, number of undamaged grains, weight of undamaged grains, weight of frass, and percent weight loss of grains caused by insects in the aforementioned types of packaging, and compared this with a fifth treatment consisting of open (unpackaged) grains infested by T. granarium larvae. We found that there was more damage to grain, greater weight loss of damaged grain, greater weight of frass, and higher percentage of weight loss of grains in unpackaged wheat, followed by opaque polyethylene and polypropylene, but not in transparent polyethylene or polyvinylchloride packaging. Microphotography after 90 d to measure the lengths of openings showed the maximum lengths in scratches and holes in opaque polyethylene, followed by polypropylene, although scratches in transparent polyethylene did not have openings for measurement. Following this, the types of packaging that were resistant to damage, i.e., transparent polyethylene and polyvinylchloride, were evaluated further for phosphine fumigation efficacy in terms of mortality of T. granarium. Fumigation results showed that in 24 h more larval mortality occurred in transparent polyethylene than in polyvinyl chloride, making transparent polyethylene overall a better foodstuff packaging material for protection from T. granarium.