Bulletin of Insectology 66 (1): 143-152, 2013 ISSN 1721-8861 Thrips abundance on sweet corn in southeastern Poland and the impact of weather conditions on their population dynamics Paweł K. BEREŚ 1 , Halina KUCHARCZYK 2 , Marek KUCHARCZYK 3 1 Institute of Plant Protection – National Research Institute, Regional Experimental Station, Rzeszów, Poland 2 Department of Zoology, Maria Curie-Skłodowska University, Lublin, Poland 3 Department of Nature Conservation, Maria Curie-Skłodowska University, Lublin, Poland Abstract In the study years, thrips (Thysanoptera) began infesting the sweet corn plants from the first (2008) or the second (2009-2010) decade of May. A single population peak was observed during pest development and it took place in the second decade of July. The pests ceased from the plants by the second (2008-2009) or third (2010) decade of September. Field and laboratory observa- tion demonstrated that thrips fed on all the aboveground plant parts. Initially, the highest number of individuals was found on the bottom side of leaves, and later in sheaths, on fresh silk, under the cob husk, and on tassels. In total, during the study years, 16 species of thrips were found on sweet corn. Anaphothrips obscurus (Muller) was found on plants at the earliest time. Frankliniella tenuicornis (Uzel) was the dominant species, and its individuals accounted for 65.4 to 80.5% of the total number of thrips col- lected in consecutive years. The second most frequent species was Haplothrips aculeatus (F.), and its share in the collected sam- ples was between 13.6 and 22.1%. H. aculeatus normally attained its population peak two weeks after the dominant species, i.e. F. tenuicornis. Weather conditions (precipitation and temperature) affected the speed of corn plant growth, date of plant infestation by thrips, and the duration of the population peak of thrips on the plants. However, weather conditions had no effect on the date of the thrips population peak occurrence, which was between 13 and 15 July in consecutive years. Key words: Frankliniella tenuicornis, Haplothrips aculeatus, population dynamics, species composition, Thysanoptera, Zea mays var. saccharata. Introduction Sweet corn (Zea mays var. saccharata [Sturtev.] L.H. Bailey) is a commercially important crop in many coun- tries. In the USA, this vegetable is grown on over 200,000 ha, and in China on about 100,000 ha. Other large producers of sweet corn are Brazil and Australia (about 40,000 ha), and Thailand (about 30,000 ha). In Europe, the leading producers of sweet corn are Hun- gary and France. In Poland, sweet corn is still grown on a small scale, but the growing consumer demand for sweet corn may lead to its increased production. The acreage of this vegetable is estimated at about 5,000 ha (FASonline, 2004; Nelson et al., 2011; Waligóra and Skrzypczak, 2012). Similarly to fodder corn (Zea mays L.), sweet corn is infested by many harmful organisms, which can lead to losses in yield and quality in case of high pest popula- tion levels. Particularly dangerous are species damaging cobs, as it disqualifies them from direct consumption or for processing industry (Waligóra, 1992; O’Day et al., 1998; Carena and Glogoza, 2004). So far, about 30 pest species, which may potentially damage plants, have been identified on fodder corn in Poland (Bereś and Pruszyński, 2008; Bereś, 2011). However, the species composition of pests infesting sweet corn has not been established. Currently, major pests of fodder corn and sweet corn in Poland include the European corn borer (Ostrinia nubilalis Hubner), and the frit fly (Oscinella frit L.), (Mazurek and Hurej, 1999; Mazurek et al., 2003; Waligóra et al., 2008, 2011). These species are also commercially important pests of sweet corn in other countries (Jepson and Mathias, 1960; Malvar et al., 2002; Bailey et al., 2005; Nelson et al., 2011). Pests of sweet corn that may influence the size and, particularly, the quality of the yield also include aphids (Aphididae) and, increasingly frequently, thrips (Thy- sanoptera) (Stoetzel and Miller, 2001; Obrist et al., 2005; Khan et al., 2006; Kucharczyk et al., 2011; Nel- son et al., 2011; Waligóra and Skrzypczak, 2012). Thrips, because of their minute size and cryptic behav- iour, are a relatively poorly studied group of corn pests, although in Poland studies on their effect on corn plants were carried out in the 1950s and 1960s (Kania, 1962a; 1962b; Zawirska, 1969). Lisowicz (2001) reported an important role of thrips as fodder corn pests and the need for their control in the southeastern part of Poland. Thrips feeding on plants usually do not have a direct significant effect on corn yield, but their indirect harm- fulness is considerably greater because the infested plants are more susceptible to pathogens infections, in- cluding fungal ones, like mycotoxin-producing Fusa- rium spp. (Parsons and Munkvold, 2010). Trzmiel and Lubik (2001) confirmed that the risk of viral diseases in fodder corn is increasing in Poland, and this may also concern sweet corn. Corn thrips (Frankliniella williamsi Hood) is the major vector of viral corn diseases, e.g. corn chlorotic mosaic virus (MCMV) in Hawaii and in Central America (Nelson et al., 2011), while western flower thrips (Frankliniella occidentalis Pergande), re- corded as the major component of Thysanoptera fauna on corn in the Czech Republic (Habuštowá and Sehnal, 2007) are able to transmit many plant viruses, as was observed in greenhouse cultivations (Cloyd, 2009). The growing interest of researchers in studies on the