Management of the Lygaeidae stink bug in soybeans

Common on sunflower plants and weeds, a small stink bug from the family Lygaeidae has caused apprehension among soybean farmers in Paraná. Still without conclusive data that can identify the potential for damage to the crop, the insect demands investigation.

From the second half of October 2016, researchers from the Entomology team at Embrapa Soja received consultations from farmers and agronomists from the central western and north central region (IBGE) of Paraná regarding the occurrence of a small stink bug in soybeans. So far in Paraná, three complaints have been registered in Jussara, two in Maringá, one in Sarandi, one in Campo Mourão, one in Arapongas and another in Londrina. Reports of the occurrence of this insect have also been common in different WhatsApp groups in which Embrapa Soja researchers occasionally participate. These reports have worried soybean farmers and led to some insecticide applications to control the pest, often hastily. 

Regarding the identification of the insect, analyzing the images received, it was suspected that it was a bedbug from the Lygaeidae family, which was confirmed by a specialist in this family, taxonomist Alexandre Silva de Paula, from the Federal University of Ouro Preto. Insects collected in Paraná are being sent to de Paula in order to identify the species. The taxonomist said that it is probably a bedbug from the Orsillinae subfamily, from the Lygaeidae family. It is a very small insect, about 4 mm long, that moves quickly and usually stays on the ground, under straw, crop residues or protected by weeds. In Paraná, the occurrence of these bedbugs of the following genera is already known. Nysius e Xionysius, which belong to the Lygaeidae family, subfamily Orsillinae, commonly associated with sunflower cultivation and various weeds of the Asteraceae family. Surveys of insects associated with soybean cultivation, carried out by Embrapa Soja over the years, indicate that this stink bug occurs in soybean crops in Paraná, however, usually at a low population density.

Some of the reports of the occurrence of the pest that reached Embrapa Soja associate the insect attack with the occurrence of yellowing and death of young soybean plants, however there may be other factors that cause death of plants in the initial phase of the development of the soybean crop, It is not possible to state that the Lygaeidae stink bug is causing significant damage to this crop. It is important to consider that during the month of October 2016 there was little rain or irregular rain with high temperatures, which may be associated with several events that cause mortality of soybean plants. In other cases, only small pits on the leaves are reported, with no dead plants. However, in most cases, injuries that could indicate any damage to soybean plants or loss of productivity were not reported. The presence of this bug in soybeans is associated with the occurrence of dark spots on the leaves and cotyledons, which are the insect's feces. Furthermore, what has caught the attention of farmers is the high population density of the stink bug in soybean crops. Some of these farmers have already applied insecticides to control this bug, despite there being no concrete evidence that its presence in soybean crops causes productivity losses to the crop. However, it is important that the interaction of this bug with soybeans is investigated in scientific detail to determine whether it has the potential to cause damage to the crop.

With the aim of determining the damage caused by this stink bug to soybeans, on November 1st, a sample of insects from Sarandi, Paraná, was received at Embrapa Soja, which were infested on soybean plants in a greenhouse. To date, symptoms of plant death observed in the field have not been observed under controlled conditions, that is, it is not yet possible to state that this insect causes death of soybean plants. Furthermore, on November 11th, researchers from Embrapa Soja visited areas infested with the pest in the Londrina region to better visualize and characterize the occurrence of this stink bug in crops. On that occasion, soybean plants exposed to the insect were collected and later analyzed in the laboratory. In these plants, insect feeding bites were observed on the cotyledons, but almost no bites were observed on the younger soybean stems or leaves. This indicates that the insect's damage potential is very low and that the producer should avoid insecticide applications at this initial stage of the crop.

Of the cases reported to Embrapa Soja to date, in two, one from Maringá and the other from Sarandi, the insect occurred with greater intensity in places infested with turnip greens, a weed from the Brassicaceae family. It is possible that this insect previously multiplied on weeds and is now using soybeans as a complementary source of food and shelter, due to the absence of preferred host plants. It is important to take into account that, when soybean is sown, the weeds present in the area are desiccated, thus eliminating practically all vegetation. However, in the case of turnip greens, the death process is slow and the plant remains with green tissue for longer than other plants before dying. This may be contributing to the greater occurrence of this insect associated with turnip greens, so it is important to verify through scientific research the role of turnip greens and soybeans in the survival of bedbugs and whether these plants allow their development and reproduction, or whether Insect multiplication depends on other plants that occurred prior to soybean sowing.

In another case, in Jussara, Paraná, the farmer reported a greater occurrence of the stink bug in places infested with marcelinha, a weed from the Asteraceae family. In this case, considering what is known so far about typical host plants for this stink bug, it is more likely that marcelinha may be effectively contributing to the multiplication of this stink bug, as well as other plants in this family, as they are considered preferential hosts.

Soybean sowing in regions where the Lygaeidae stink bug has been observed occurred in late September and early October and farmers have observed the insect in late October, approximately four weeks after soybean emergence. In this context, it is important to investigate each case in detail in order to analyze whether the stink bug is remaining on a weed plant or whether it is actually attacking soybeans. In the initial observations made by Embrapa Soja researchers, these insects are attacking more the cotyledonary leaves and little attack can be observed on the stems or younger leaves of the soybean, indicating that the oilseed may be serving more as a supplementary source of water. Also, reports from Emater (MIP/MID Weekly Newsletter from 6 to 11/11/2016) indicate that crops in Jussara, Maringá and Sarandi where the occurrence of infestations of this bug had been reported, that week no more specimens were found in the field .

With the development of soybean plants, the occurrence of this insect will possibly become less important. Therefore, caution is necessary when making control decisions, as insecticide applications at this stage of the crop may cause mortality of important biological control agents, unbalancing the agrosystem, and may lead to more problems with pests due to the elimination of these beneficial arthropods. Furthermore, the products most used to control these “bugs” are the same ones used for soybean pests during the reproductive period. The early use of these products can further aggravate the problems with soybean bug resistance to the insecticides used, which is already observed today. Therefore, the recommendation is that the producer does not prematurely apply insecticide to control this pest since the soybean plant is quite tolerant to this “bug”. In the literature, damage from this pest is only reported when overpopulations migrate from weeds to crops of agronomic importance. Therefore, an agronomist should always be consulted to better assess the area and correctly adopt pest and weed management on your property.

 

Samuel Roggia, Edson Hirose, Daniel Ricardo Sosa-Gomez, Adeney de Freitas Bueno, Embrapa Soja; Beatriz Spalding Corrêa-Ferreira, retired researcher at Embrapa Soja

Article published in issue 211 of Cultivar Grandes Culturas.