New discovery of the oldest maize weevils in the world from Jomon potteries, Japan

The maize weevil (_Sitophilus zeamais_) and rice weevil (_Sitophilus oryzae_) are two of the most damaging insects for stored grains, and are characteristic species of ancient Japan. Both species and the granary weevil (_Sitophilus granarius_) are common elsewhere in the world, but the natural distribution of maize and rice weevils is restricted to the Old World^1^. Japanese archaeological records contain a few maize weevil fossils after the Middle Yayoi period (ca. 2000 aBP)^2^. However, since evidence of weevils was discovered as impressions in Jomon potsherds in 2004^3^, many weevil impressions have been found. The oldest is from the Late Jomon (ca. 4000 to 3200 aBP). These findings and other archaeological evidence suggest that the maize weevil invaded Japan from Korea, accompanying the spread of rice cultivation^4^. However, in 2010 we discovered older weevil impressions dating to ca. 9000 aBP. These specimens are the oldest harmful insects discovered from archaeological sites around the world. The new discovery is valuable for future entomological research because such specimens are absent from the fossil record. It is also archaeologically and culturally interesting because this provides evidence of harmful insects living in Jomon villages. However, the new discovery raises the question of what these weevils infested: did cereal cultivation exist 9000 years ago? We have no persuasive answer, but hope one will be provided by future interdisciplinary collaborations among geneticists, entomologists, and archaeologists

A New Light on the Evolution and Propagation of Prehistoric Grain Pests: The World's Oldest Maize Weevils Found in Jomon Potteries, Japan

Three Sitophilus species (S. granarius L., S. oryzae L., and S. zeamais Mots.) are closely related based on DNA analysis of their endosymbionts. All are seed parasites of cereal crops and important economic pest species in stored grain. The Sitophilus species that currently exist, including these three species, are generally believed to be endemic to Asia's forested areas, suggesting that the first infestations of stored grain must have taken place near the forested mountains of southwestern Asia. Previous archaeological data and historical records suggest that the three species may have been diffused by the spread of Neolithic agriculture, but this hypothesis has only been established for granary weevils in European and southwestern Asian archaeological records. There was little archeological evidence for grain pests in East Asia before the discovery of maize weevil impressions in Jomon pottery in 2004 using the “impression replica” method. Our research on Jomon agriculture based on seed and insect impressions in pottery continued to seek additional evidence. In 2010, we discovered older weevil impressions in Jomon pottery dating to ca. 10 500 BP. These specimens are the oldest harmful insects in the world discovered at archaeological sites. Our results provide evidence of harmful insects living in the villages from the Earliest Jomon, when no cereals were cultivated. This suggests we must reconsider previous scenarios for the evolution and propagation of grain pest weevils, especially in eastern Asia. Although details of their biology or the foods they infested remain unclear, we hope future interdisciplinary collaborations among geneticists, entomologists, and archaeologists will provide the missing details

Details of the maize weevil impressions from the Sanbonmatsu (SBM) site.

<p>Samples are illustrated in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0014785#pone-0014785-g002" target="_blank">Figure 2</a>.</p

Diagnostic characteristics used to identify the weevil species.

<p>Three species of weevils (<i>Sitophilus zeamais, Diocalandra elongata</i>, and <i>Paracythopeus melancholicus</i>) are distinguishable by the ratio of thorax to elytron length. Another diagnostic criterion that distinguishes <i>S. zeamais</i> from <i>P</i>. <i>melancholicus</i> is the elytron end. Elytron does not cover the full abdomen in <i>S. zeamais</i> but extends the full length of the abdomen in <i>P</i>. <i>melancholicus</i> (bottom row of photographs).</p

Details of the maize weevil impressions that were obtained at Jomon sites by 2009. Numbers correspond to the photographs in Figure 1.

<p>Japanese academic circles on archaeology divides the Jomon Period into six phases as followings. The Incipient Jomon (15000-11500BP), The Earliest Jomon (11500-7300BP), The Early Jomon (7300-5500BP), The Middle Jomon (5500-4500BP), The Late Jomon (4500-3300BP), The Latest Jomon (3300-3000BP).</p

CT scan images of the impression of maize weevil SBM0024 from the Sanbonmatsu Site.

<p>The CT scan images show details of the insect's legs, rostrum end, and antennae that were previously unseen in the impression replicas. These findings demonstrate the method's superiority to the older impression replica method for correctly identifying insects. The lengths in this example are 0.908 mm (rostrum), 1.246 mm (thorax), and 1.934 mm (abdomen). In our experiment, the shrinkage of the mean lengths from the original maize weevils to the impressions they left in potsherd cavities were to 92.16% (rostrum), 91.63% (thorax), and 96.28% (abdomen) of the original length. Therefore, the original lengths would have been 0.985 mm (rostrum), 1.359 mm (thorax), and 2.008 mm (abdomen). These lengths are larger than those of modern reference specimens that were reared in cleaned rice grains (mean lengths of rostrum, thorax, and abdomen were 0.903, 1.105, and 1.428 mm, respectively; <i>n</i> = 20) and were similar to the size of weevils reared in chestnuts (mean lengths of rostrum, thorax, and abdomen were 1.056, 1.338, and 1.882 mm, respectively; <i>n</i> = 20).</p

Scanning electron micrograph (SEM) images of impression replicas of maize weevils that had been discovered at Jomon sites by 2009.

<p>The 37 weevil impressions were discovered in Jomon pottery, mainly collected on Kyushu, dating to between ca. 4500 and ca. 3000 BP. Based on the diagnostic criteria described in the text, including the size of the specimens, these weevils appear to have been maize weevils (<i>Sitophilus zeamais</i>). Details of each impression replica are provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0014785#pone-0014785-t001" target="_blank">Table 1</a>.</p