The clonal propagation of Eucalyptus in plastic tube containers with growing media substrate without direct contact of seedlings with soil floor in nurseries, changed the status of insects such as fungus gnats (Diptera: Sciaridae), especially the genus Bradysia. This study monitored populations of adults within a Eucalyptus spp. tree nursery in the municipality of Bom Despacho, Minas Gerais State, Brazil. Adult samples were collected monthly, during seven months with 58 yellow sticker card traps, including 21 in a mini garden, 16 in a greenhouse, and 21 in a shaded house. All individuals collected were identified as Bradysia difforrais Frey, 1948 (= Bradysia paupera Tuomikoski, 1960) (Diptera: Sciaridae). The population dynamics of B. difformis were not correlated with environmental variables (temperature and relative humidity), but varied between the three sectors of the nursery. This work is the first report associated with the population dynamics of B. difformis of nursery seedlings of Eucalyptus spp. in Brazil.
Clonal propagation of Eucalyptus in plastic tubes with growing media substrate in nurseries may provide favorable environmental conditions which allow insect species to become pests (Berti Filho & Wilcken 1993) including fungus gnats. Fungus gnats (Bradysia spp.) (Diptera: Sciaridae) are the main group of insect pests in nurseries and commercial greenhouses of many countries (Cloyd & Zaborski 2004).
Adults of B. difformis are dark brown to black, 1.8–2.1 mm long, with a large number of dark bristles on the body, thorax and abdomen, dark brown to clear yellow lateral areas, with elongated and grayish thin legs and wings (Menzel et al. 2003). Adults live about 2 to three days and females lay 30 to 70 eggs beneath the soil gowing media substrate. The larvae have four instars with a lifecycle of 21 days (Harris et al. 1995) depending on the temperature, substrate and host plant (Bravo et al. 1993). The fungus gnat B. difformis is often found in greenhouses where it can become a pest.
In Brazil, larvae of Bradysia coprophila Lintner (Diptera: Sciaridae) damage Eucalyptus spp. seedlings in nurseries (Zanuncio et al. 1996), where they feed on fungi and roots, causing damage that reduces crop biomass and water absorption (Vaughan et al. 2011) primarily in moist substrates with high organic matter content (Dreistadt 2001). This can compromise growth and development of Eucalyptus cuttings by causing a loss of vigor, and yellowing of its leaves (Berti Filho & Wilcken 1993) and make plants susceptible to attack by pathogens (Gardiner et al. 1990). Larvae of this insect feed on living plant matter, especially roots (Werner Mohrig, pers. commun.). Bradysia difformis like B. coprophila feeds on roots of Eucalyptus and Pinus in nursery seedlings in South Africa (Hurley et al. 2007).
Gnats of the genus Bradysia can spread pathogens both directly or indirectly (Gillespie & Menzies 1993; Shamshad et al. 2009) such as Verticillium albo-atrum Reinke & Berthold in alfalfa (Kalb & Millar 1986), Botrytis cinerea Persoon ex Fries and Colletotrichum fragariae Brooks in strawberry (Radin et al. 2009), B. cinerea and Fusarium spp. in conifer seedlings (James et al. 1995) and Pythium aphanidermatum (Edson) Fitzp. in cucumber (Jarvis et al. 1993).
The objective of this study was to monitor populations of the genus Bradysia in a Eucalyptus spp. nursery to improve fungus gnats information in commercial seedlings production in Bom Despacho, Minas Gerais State, Brazil.
MATERIALS AND METHODS
This work was developed in a Eucalyptus seedling production unit, that produces eight million Eucalyptus seedlings/year, in the Municipality of Bom Despacho (S 19° 44′ 11″ W 45° 15′ 08″ at an altitude of 769 m asl), Minas Gerais State, Brazil.
Adults samples were collected monthly with 58 card-type yellow sticker traps suspended 10 cm from the top of Eucalyptus seedlings. Twenty-one traps were distributed in an area of 4,900 m2 of a clonal mini garden (MJC) sector with three traps systematically distributed between hydroponics channels; 16 in 2,675 m2 in greenhouses complex (CV) sector with 2 traps systematically distributed in the center of each vegetation house) and 21 in a 5,922 m2 area of the shaded area (CS) sector with 2 traps systematically sampled per vegetation. Traps were monitored during seven months in CS-shade house, the CV- greenhouse and MJCmini clonal garden.
The traps were removed each month and sent to the Laboratory of Forest Entomology of the Federal University of Lavras in Lavras, Minas Gerais State, Brazil where the fungus gnat adults per trap and nursery sector (CS, CV and MJC) were counted with the help of a stereoscopic microscope (40x).
Traps were not collected at the CV in July and August because this area was cleaned and seedlings were removed of the greenhouses. Adults were removed from the traps with a brush and liquid petrolatum in the first month of monitoring and sent to Prof. Dr. Werner Mohrig for identification.
Temperature data (°C) and %RH were obtained with digital thermometer-hygrometer monitors in CS, CV and MJC sectors of the seedling production unit and correlated with the number of fungus gnats per sector of the nursery (Pearson, P ≤ 0.05).
Generalized linear models (GLM), using the appropriate distribution of errors for the number of insects as response variable (Crawley 2005), were constructed to test the effect of CS, CV and MJC sectors of the nursery and month on population number of B. difformis and the means per sector compared by analysis of contrast models (P < 0.05) (Ferry et al. 2009). Statistical analysis was performed with the program R (R Development Core Team 2008) and the package Multicomp (Hothorn et al. 2008).
All insects collected were identified as Bradysia difformis Frey, 1948 (=Bradysia pauper Tuomikoski, 1960) (Diptera: Sciaridae). The average number of individuals of B. difformis was not correlated with temperature and relative humidity in the clonal mini garden (Temperature (°C): 26.6 ± 0.24, r = 0.4716, P = 0.2854, RH (%): 53.1 ± 0.84, r = -0.0695, P = 0.8823), greenhouse (Temperature (°C): 29.3 ± 0.27, r = -0.7695, P = 0.1281, RH (%): 80.7 ± 0.76, r = 0.6304, P = 0.2542) and the shaded area (Temperature (°C): 26.5 ± 0.24, r = 0.6269, P = 0.1319, RH (%): 53.1 ± 0.84, r = -0.1687, P = 0.7176) sectors.
The numbers of B. difformis captured varied between the 3 sectors in each month evaluated. Nine to 83 gnats per trap were collected in the CS, CV and MJC sectors of the nursery and the MJC generally had the lowest number of fungus flies (Fig. 1). The GLM analysis using contrasts showed differences in the number of adults of B. difformis in each sector of the nursery but without similarity between populations of these gnats per sector in any month evaluated (Fig. 1).
The temperature and relative humidity affected the population dynamics of B. difformis; and the numbers of Bradysia adults which was positively correlated with temperature and humidity (Prishchepa & Kondratenko 2008). Contrary in this study, the abundance of B. difformis was not correlated with temperature and relative humidity, probably because the environmental variables were evaluated in a shorter time period (seven months).
Bradysia difformis is expected to have higher frequency numbers in CV and CS sectors, compared to the MJC, due to the substrate of this sector which consists only of sand and this species is found normally in substrate with organic matter (Dreistadt 2001), despites similarity in environmental conditions. The constant moist growing media substrate for propagating Eucalyptus seedlings combined with an average temperature of 27.5 °C in nursery areas may provide ideal conditions for growth and development of the fungus gnat independent of production and seasonal environmental conditions since the optimum temperature for adult emergence of species of this group is above 20 °C (Lee et al. 1998).
This is the first report of B. difformis monitored in one commercial nursery with Eucalyptus spp. seedlings in Brazil. The abundance of this insect was not correlated with environmental conditions, probably because these variables were evaluated in a shorter time in this study. However, the number of individuals of this insect varied between different sectors of the Eucalyptus nursery.
To Prof. Dr. Werner Mohrig for species identification and biological information about Sciaridae family. To “Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)”, “Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)” and “Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG)” for financing support. Asia Science edited and corrected this manuscript.