In the rice fields of the Delta, one of the most damaging pests is the rice borer (Chilo suppressalis). It is a lepidopteran (family Crambidae) of crepuscular habits, whose larvae perforate the canes to feed on the internal tissues of the rice plant. This involves a significant weakening of the affected plants, in such a way that the spikes are significantly less productive than those of the healthy plants.
The rice borer develops up to 3 generations throughout the year. Between the end of April and the middle of June, coinciding with the planting and the initial stages of vegetative development of the rice plants, the first adults of the season (1st flight) appear, which come from larvae that, mostly, will have been hibernating inside reeds of reed plants (Phragmites sp.). These adults give rise to the 1st generation of larvae.
During the month of July and until the beginning of August a second flight of adults is detected, origin of the 2nd generation larvae. A good part of these larvae develop parallel to the flowering and spreading of the rice and, from the end of August, they give rise to a third flight of adults.
Both the attack symptoms and the damage caused by the rice borer are different depending on the stage of development of the rice plant. In any case, these are originated by the larvae, which are the ones that perforate the canes to feed on the corresponding internal tissues.
It is frequent that the attacked plants are arranged in circles or small groupings. This is because, normally, the larvae do not make large displacements with respect to the plant where the laying has been made and only change cane when they need more food or look for a suitable cane to crisalidate. Rice in the vegetative state is attacked by larvae of the 1st generation in the month of June, and the clearest symptom is the appearance of reeds with a dry central leaf. At this moment, the losses are minimal since the plant usually has the capacity to take out shoots and compensate the affected canes. Anyway, this level of population will influence future attacks.
In flowering and at the beginning of the spike, the attack of 2nd generation larvae usually occurs, with the consequent characteristic appearance of white spikes (panicles), although this symptom is not exclusive to attacks by borers. It is the most serious attack, since the affected plants can end up obtaining very low productions.
During the spike, with the pasty and hard grained rice, the rice is attacked by 2nd and 3rd generation larvae. The symptoms of the attack of the borer in this stage are plants with more vertical spikes than the rest due to the relatively low weight of the rice grains. This attack can also affect the cane itself, since sometimes it does not have enough strength to support the weight of the plant and falls down. In this case, if the spikes are inside the water the grains will deteriorate (germination in the same panicle), at the same time as mowing is difficult since it is harder for the machines to pick up this rice.