بررسی خسارت‌زایی سوسک برگ‌خوار غلات Oulema melanopus (L.) (Coleoptera: Chrysomelidae) در مزارع گندم

Background and Objectives
Cereal leaf beetle, Oulema melanopus (L.) (Coleoptera: Chrysomelidae), is an important world-wide pest of wheat and barley. Adults and larvae feed on the upper side of leaves and chew long strips of tissue between the leaf veins especially on flag leaf, which can reduce grain yield. However, the greater damage is usually caused by feeding activity of larvae rather than adults. This paper presents the results of an investigation on the damage caused by this pest in wheat fields.
Materials and Methods
This study was carried out in three wheat (Line: N8019) fields located at Neka, Sari, and Joybar, in Mazandaran province, Iran. Occurrence and activity of the pest in the fields was monitored weekly from mid-February until mid-May during 2013 and 2014. Coinciding with the end of larval feeding-activity (early May), 10 spots were selected in each field, and 10 beetle-infested wheat stems (showing pest damage symptoms on flag leaf surface) and 10 intact stems (without damage symptoms) in each spot were randomly selected and labeled. The flag leaves of infested stems were photographed and infestation rates (% leaf surface damage) were assessed and categorized in four groups (<10%; 10-20%; 20-40%; >40%) using UTHSCSA image tool for windows version 3. At harvest time, the spikes of all labeled stems (infested and intact) were separately collected and grain weight and grain number per spike as well as 1000-kernel weight was measured. The relationship between percentage of flag leaf damage and yield (grain number and grain weight per spike) and 1000-kernel weight was calculated using regression analysis.
Results
Overwintered adult beetles were observed in the fields from early March, with a peak activity occurring at mid-March. Larval feeding activity started during late March and continued until early May. In both sampling years, the highest grain weight loss and number of grains per spike, and 1000-weight loss were observed in the >40% of infestation rate. Grain weight loss, number of grains reduction per spike, and 1000-kernel weight loss in the >40% infestation rate were 42.58%, 25.86% and 32.76% in 2013 and 32.42%, 15.59% and 19.82% in 2014, respectively.
Discussion
Results showed that the infestation rate of flag leaves can negatively affect grain weight per spike and 1000-kernel weight. With increasing percentage of flag leaf infestation, grain weight and grain number per spike, and yield decreased. However, in this study the infestation rates of flag leaves were mostly <10% or 10-20% and rarely reached >40% rate. During two-year sampling, only 10% out of 566 beetle-infested stems showed more than 40% leaf surface damage. In contrast, 46% and 62.3% of collected samples in 2013 and 2014, had less than 20% flag leaf surface damage. Therefore, the probability of severe damage caused by O. melanopus was low to moderate in the study sites.