An investigation of peanut storage pests in Queensland. 1. Introduction, species arad pest status

Abstract

The arthropods associated with groundnuts stored in the shell were investigated in Queensland in 1961-62, primarily at a place in the south where about 80% of the total crop from the State (normally over 20, 000 tons) is stored. The crop is put into store during March-September, and when the concrete bins, which can accommodate 12, 000 tons, are full, the rest is stored in bags stacked on dunnage under temporary roofing in the open until silo space is available. The species found during the survey and in earlier and subsequent investigations are listed, and their abundance indicated, in an appendix. In the bag stacks, infestation was only light or moderate and did not significantly affect the quality of the groundnuts unless high proportions of them were cracked or broken. The principal species involved were Tribolium castaneum (Hbst.), Oryzaephilus mercator (Fauv.), Cadra cautella (Wlk.) and sometimes Plodia interpunctella (Hb.). Fumigation with methyl bromide combined with preventive measures is considered sufficient for protection. In the bins, C. cautella and sometimes P. interpunctella were the first species to become established. They reached peak numbers in January-February, but mortality among the larvae of the succeeding generation caused possibly in part by Mattesia dispora, which infected almost all the larvae in late summer, but also by the parasite Bracon (Microbracon) hebetor Say and the predators Xylocoris flavipes (Reut.) and Melichares tarsalis (Berl.), was high. T. castaneum and O. mercator appeared soon after C. cautella and increased rapidly; numbers were highest at the surface. Species of Cryptolestes, notably C. pusilloides (Steel & Howe), which was first recorded on groundnuts in 1961, were common but became numerous at the surface only where general populations were high. Damp conditions favoured Carpophilus dimidiatus (F.), and large numbers remained in groundnuts adhering to the walls when the bins were emptied: gamma BHC (lindane), dieldrin and malathion applied as fogs were of no value against it. The most important losses were probably the result of direct damage to the kernels of cracked and broken groundnuts, which impaired their quality. In addition, the heating and increased moisture associated with insect infestation favoured fungus growth, which discoloured the shells and reduced the percentage of high-quality groundnuts. The inclusion of groundnuts with moisture contents above 12% quickly created conditions favourable for insect attack. Recommended control measures comprise aeration of the bins, possibly combined with the application of sprays of a persistent insecticide. From a consideration of the number of hours per day throughout the year in which the humidity exceeded 85%. and temperatures were below each of 12 values between 40 and 85°F., it appeared that bulk temperatures could be maintained at 65°F. in October, 60°F. during April-September and 70°F. or less during the critical period of November-March; this would considerably reduce insect activity. Initial control would be facilitated by filling the bins in winter. By aerating the groundnut bulks, temperatures would be equalized throughout them and the accumulation of moisture prevented; moisture could be similarly equalized by including some groundnuts with high moisture content among drier ones. The use of persistent insecticides as a short-term measure is considered preferable to the current practice of fumigating with methyl bromide when infestation becomes severe.