The commodity weight loss resulting from insect feeding is influenced by the suitability of commodity as a diet, insect density or age structure and the duration of infestation. Table 1 Weight loss gives the weight loss caused by T. castaneum larvae and adults feeding on 32 commodities as reported in 9 studies. Commodity weight loss was estimated for 19 commodities that were not cereal grains (Hamed and Khattak 1985, Nwawa 1993 and Rajput et al. 2016). Excluding the survey, the weight loss estimates averaged 6.8% for non-cereal commodities (range 1.0 to 15.6%, n=22) and 3.4% for cereal grains and their products (range 0.2 to 18.8%, n=55).
The methods for determining commodity weight loss and damage differ sufficiently between studies that comparisons are likely to be most reliable within a study. Temperature and moisture content can influence T. castaneum feeding and commodity weight loss. Increasing temperature by 4oC from 28 to 32oC increased weight loss of whole grain sorghum by 2-fold and a further increase by 3oC from 32 to 35oC decreased weight loss by 1.76-fold (Majeed et al. 2016). With these temperature changes, damage increased 1.9-fold and decreased 1.8-fold. For barley, increasing temperature by 4oC from 28 to 32oC increased weight loss by 2-fold and a further increase by 3oC from 32 to 35oC decreased weight loss by 1.93-fold (Javed et al. 2016). A 2% increase in moisture content of whole grain wheat from 12.2 to 14.2 increased insect feeding damage by 1.6- fold during the first 60 days, 1.3-fold during next 60 days and 1.2-fold during the last 60 days of insect infestation (Daniels 1956).
For grain, weight losses caused by T. castaneum feeding were highest for wheat followed by rice then maize (Ali et al. 2016). Weight loss for wheat was 1.5-, 1.3- and 1.4-fold greater than for rice and 6.0-. 2.4- and 1.7-fold greater than for maize after 30, 60 and 90 days of insect infestation. Weight loss for rice was 4.0-, 1.9- and 1.2-fold greater than for maize after 30, 60 and 90 days of insect infestation. Ali et al. (2016) used 80% whole grains and 20% cracked grain and had the highest insect populations.
Majeed et al. (2016) found that percentage of sorghum kernels damaged was 2.47 times higher than the % weight loss caused by T. castaneum feeding. The percent commodity damage by insects was even higher in a survey of 15 farmers (Ajay and Rahman 2006). On cereal grains over 30 or 60 days red flour beetle population density increased 0.9- to 1.6-fold during studies.
During development, a T. castaneum larva consumes a total of 13 mg of wheat flour and adults during their lifetime consumes 315 mg of wheat flour (Hagstrum and Subramanyam 2000). Over lifetime, T. castaneum consumed 2-fold more than Rhyzopertha dominica (Fabricius) and 23-fold more than Cryptolestes ferrugineus (Stephens). After eating the germ of a wheat kernel, a larva feeds on the endosperm and may consume most of the kernel (Daniels 1956). In addition to visible feeding damage to 1.5 wheat kernels, larva may inflict less visible damage to another 21.1(Mookherjee and Khanna 1971). During adult life, the entire germ of 7–12 wheat kernels can be consumed (Karunakaran et al. 2004). A larva consumed a mean of 7.9 mg of millet seed at 70% rh and 12.3 mg at 10% rh damaging germ of an average of 18 and 26 kernels, respectively (Roorda et al. 1982). Larvae prefer the germ of peanuts and tend to wander from kernel to kernel during their development (Applebaum 1969). Brazil nuts are damaged by adults and larvae starting as a scratched surface, evolving into galleries and even modifying the shape of the nut (Pires et al. 2017).
Starting with densities of 1, 2 or 3 pairs of adults per 50 grams, T. castaneum populations damaged higher percentages of wheat kernels over 6 months at 30oC and 9 .7 and 15.5% wheat moisture content than Sitophilus oryzae (Linnaeus) and Trogoderma granarium Everts populations (Ramzan and Chahal 1985). Damage was significantly higher at 15.5% wheat moisture content than at 9.7%.
In addition to weight loss and damage to commodity, contamination by insects and reduced baking quality also may be important. In the US, Food and Drug Admistration (FDA) regulates the number of fragments in milled grain and several methods have been developed to determine the number of fragments (Bhuvaneswari et al. 2011). Semolina was investigated because T. castaneum fragments in noodles made from it can be undesirable dark spots. Red flour beetles produce quinones that can give commodities off-odor or -taste and may be carcinogens at level much higher than those resulting from most flour beetle infestations (Stejskal and Hubert 2006). Most of the research on flour beetles as source of allergens has been done with Tribolium confusum, but Popa et al. (1970) reported on T. castaneum as source of allergens. Very large T. castaneum infestations can increase fat acidity, decrease thiamine, reduce the quality of gluten and decrease loaf volume of bread baked with infested flour (Venkatrao et al 1960, Smith et al. 1971). They may reduce nutritional value of cashew kernels and contaminate them with uric acid (Prabhakumary and Sini 2008).
Tribolium castaneum also can influence fungal, bacterial and tapeworm problems in grain or animal feed and at bakeries. Yun et al. (2018) found that T. castaneum can spread fungal contamination in stored rice and may consequently increase the mycotoxin problem. Badu-Yeboah (1995) in the laboratory showed that Aspergillus flavus conidia adhered to the bodies of T. castaneum, were transported through maize and persisted in their gut. Aspergillus fumigatus, Cladosporium herbarum, Penicillium citinum and P. purpurogenum also were isolated from gut. Channaiah et al (2010) demonstrated that T. castaneum can vector antibiotic resistant Enterococcus faecalis in cattle and poultry feed. In a bakery, T. castaneum can be an intermediate host for a tapeworm and can infect man if the infected beetle is eaten (Makki et al 2017). This is unlikely to be a problem if the primary rat host is controlled.
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