Control of the caterpillar in beans

High infestations of the caterpillar, common in soybean and cotton crops, have been detected in castor and bean plants in São Paulo. The presence of different phases of the insect's biological cycle in the two crops indicates that the pest is adapted to colonize crops, requiring greater attention from producers.

Known as the false caterpillar, Chrysodeixis includens (Walker) (Lepidoptera: Noctuidae) is an insect whose geographic distribution ranges from the north of the USA to the south of South America, being more restricted to the Western Hemisphere. Previously referred to as Pseudoplusia includens, today considered a synonym, in Brazil it occurs from Rio Grande do Sul to Roraima.

It is a polyphagous insect, having as hosts 73 plant species, belonging to 29 different botanical families. Among the plants of economic interest attacked by this insect, soybeans, cotton, beans, tobacco, sunflowers and vegetables stand out.

The caterpillars of C. includens They are light green in color, with longitudinal white stripes and black dots on the back. They go through six larval instars and, when fed with soybean parts, they complete the immature phase in around 15 days. They are easily differentiated from the caterpillars of other noctuids, due to the presence of three pairs of false legs in the abdominal region, meaning that, when moving, there is intense movement of the body. This behavior, simulating measuring palms, led to them being called “measurers” or “measure-palm”. In the pre-pupa stage there is a marked change in color, which becomes a uniform yellowish-green. The pupal stage occurs in the leaves, with the caterpillars wrapping themselves around these structures and weaving a thin silk cocoon. The color of the pupa varies from brown to green, with the pupal period lasting seven to nine days until the adults emerge. The longevity of adults is also very variable, ranging from ten to almost 20 days. Females can lay around 150 eggs to two thousand eggs, the vast majority of which are deposited on plants by the seventh day. The number of generations of the false pygmy per harvest varies depending on the range of existing hosts and also depending on latitude. In the United States, three to four generations are generally recorded per year. However, for Brazilian conditions, where potential hosts are available throughout the year, the number of generations may be higher.

As for their eating habits, in soybeans and cotton the caterpillars C. includens They feed on leaves located in the lower third of the plants. In their first instars, the caterpillars select the most tender leaves, feeding on those with a small amount of fiber, becoming less demanding as they develop. Until the third instar, the caterpillars leave the epidermis regions intact. However, from the fourth instar onwards, they consume large areas, preferably of older leaves, whilst keeping the main veins intact, which gives a characteristic lacy appearance to the attacked leaves, different from the damage caused by other defoliators. Between the fourth and sixth instars, caterpillars have the greatest potential for damage to crops, consuming 95% of all food necessary for the immature phase. Some studies report that, of the total food ingested in the young phase, 50% is obtained in the last two days of larval development. Although the insect is predominantly a defoliator, it can also occasionally feed on reproductive structures. This species is favored by dry conditions.

In Brazil, C. includens it was treated for several decades as a secondary pest for soybean crops, as it always infested crops of this legume in low populations. However, the abusive application of fungicides, associated with the use of pesticide mixtures, has reduced the population of this insect's natural enemies, favoring its proliferation within crops and migration to other crops. In common bean (Phaseolus L.), the pest is growing in importance in some states such as Goiás, São Paulo, Paraná and Santa Catarina, due to the damage caused in recent harvests. Although Brazil is the largest world producer of grains of this legume, with cultivation areas distributed from North to South, the attack of defoliating insects such as C. includens It can compromise productivity in the field, requiring attention from producers. However, To date, there have been no reports of this pest causing damage to castor beans in Brazil. In this country, the rise of the castor bean plant was due to the high adaptive capacity of the species and the versatility of the oil extracted from the seeds by the ricinochemical industry, driven mainly by the advent of biodiesel in the search for alternative energy sources. Despite its significant production, Brazilian productivity is still considered low in relation to other countries, and this deficiency is, in part, associated with pest attacks. In this crop, records of defoliating pests include several species of caterpillars, but none of them make reference to C. includens. In Brazilian literature, the main caterpillars related to castor beans are threadworm [Agrotis ipsilon (Hufnagel)], leaf caterpillars Lophocampa citrina (Sepp.), cosmioides spodoptera (Walk.), Rothschildia jacobaeae (Walk.), R. arethusa arethusa (Walk.), Oligocentria violascens Herrich-Schaffer and Anacraga citrinopsis Dyar, beyond the caterpillar Pyroderces riley (Wals.).

As of 2009, growing populations of C. includens have been verified in crops of common bean, cultivar Pérola and castor bean cultivar IAC-2028 located in production areas of the Lageado Experimental Farm (22º51’S, 48º26’W 740 m), belonging to FCA/Unesp in Botucatu (SP). In beans, defoliation close to 100% was observed in the field, in addition to the lacy appearance (preserved veins), similar to what occurs in soybean plants. Infested castor bean leaves showed typical and variable damage, with up to 50% defoliation in some plants. Characteristic caterpillars, pupae and adults were observed in various plant structures from both crops.

Specimens of the specimens (caterpillars and pupae) were collected from bean and castor plants and taken to the laboratory (T = 25 ± 2ºC; RH = 70 ± 10% and photophase = 12h), where they were kept until the emergence of adults occurred, to confirm the species.

The levels of defoliation observed in both crops are significant and could compromise performance and productivity rates in future harvests, indicating the need to adopt control strategies. The presence of different phases of the insect's biological cycle in both crops indicates that the pest is adapted to colonize crops, requiring greater attention from producers.

Edson Luiz Lopes Baldin, FCA/Unesp; André Luiz Lourenção, IAC

Article published in issue 197 of Cultivar Grandes Culturas.