Consequences of stress induced by heat on fitness of Delphacodes kuscheli and its yeast‐like endosymbionts

The planthopper Delphacodes kuscheli is the main vector of Mal de Río Cuarto virus in Argentina, disease that severely affects maize production. In this study, we investigated the effects of heat stress on fitness traits and on the number of its obligate yeast‐like symbionts (YLS). The exposition of newly‐hatched nymphs to 35°C for 3 days, a well‐known procedure used to reduce the number of YLS in planthoppers, was applied. To compare different fitness components between control and heat‐treated insects, we estimated nymphal instars development time, nymphal survival, adult body length, longevity, fecundity and fertility. Also, correlates of fitness, as proportion of sexes and wing forms of the emerging adults, were evaluated. In heat‐treated group, the nymphal developmental time increased due to an increase in the fifth instar duration, and the nymphal survival, body length of adults and fecundity were reduced when compared to control. There was a significant association between treatments (control and heat‐treated insects) and wing morphs. The heat treatment successfully reduced the number of YLS in third instar nymphs of D. kuscheli. Our results revealed the negative effect of heat stress on development, survival and reproduction of D. kuscheli and on the load of its YLS endosymbionts suggesting that YLS could play a crucial role in the development and reproduction of these planthoppers.     

Systemic fungicides reduce the abundance of yeast-like symbiotes and the performance of the planthopper Delphacodes kuscheli (Hemiptera: Delphacidae)

Delphacodes kuscheli is the main vector of Mal de Río Cuarto, the most important viral disease of maize in Argentina. This planthopper harbours obligate fungal mutualists, known as yeast-like symbiotes (YLS), which play a key role in the host life cycle. Here, we evaluated the efficacy of six systemic fungicides, prothioconazole (P), prothioconazole & trifloxystrobin (PT), pyraclostrobin & epoxiconazole (PE), pyraclostrobin, epoxiconazole & fluxapyroxad (PEF), picoxystrobin & ciproconazole (PC), and tebuconazole (T) on the reduction of the abundance of YLS in nymphs and newly emerged adults, and the impact of such reduction on the vector performance. All fungicides, except PE, reduced the number of YLS in nymphs treated from 3rd instar five days after starting treatments. When 3rd instar nymphs were allowed to reach adulthood, the fungicides P, PT and PC caused the highest nymphal mortality, significantly reduced the number of YLS in newly emerged adults, lengthened development time and negatively affected morphometric variables of females. There was also a trend towards a higher occurrence of brachypters. In females, the fungicides P, PT and PC caused a greater reduction of YLS when nymphs were treated to these fungicides from 3rd instar compared with nymphs treated from 4th instar. In males, the YLS number was significantly less when nymphs were fed on plants treated with P, PT, PC, and T in 3rd instar. This showed that fungicide treatments caused a greater reduction of YLS when they were applied in early stages of development. Our results provide an effective method to reduce the abundance of YLS in D. kuscheli, contributing to understand the ecological role that these symbionts could be playing in the success of this maize planthopper pest.     

Spatial pattern of the Río Cuarto corn disease vector, Delphacodes kuscheli Fennah (Hom., Delphacidae), in oat fields in Argentina and design of sampling plans

The spatial pattern of the Río Cuarto Corn Disease vector, Delphacodes kuscheli (Hom., Delphacidae), was analysed in oat fields within the endemic area of the disease, during the growing seasons 1993 and 1994. The spatial pattern was analysed by fitting the probabilistic models Poisson and negative binomial and estimation of single-date and overall aggregation indices. The population of the different stage classes, sex, and wing forms showed a significant trend to aggregation as the negative binomial model fitted the observed frequency distributions in more than 78% of the cases (sampling dates) while the Poisson model fitted well in only 28% of cases or less. Single-date aggregation index, C _A, ranged from 0.3 to 0.84. Overall (whole season) aggregation index, C _A*, estimated through the Bliss and Owen's regression method, ranged from 0.18 (female adults) to 1.08 (nymphs I–II), indicating a moderate degree of aggregation compared with other planthopper species. There were no significant relationships between aggregation and population density. The minimum number of sampling units and critical lines for sequential sampling plans were calculated based on the estimation of C _A* for the precision levels ( D ) 0.1, 0.2 and 0.3. Even low degrees of aggregation, like that of adults, demand much more sampling effort than randomly distributed populations, particularly at high densities. General implications and limitations of the proposed sampling plans for monitoring the vector population abundance are discussed.