Wheat crop pests: aphids

Wheat crops are infested by several insects and other phytophagous arthropods throughout their development, including aphids, grubs, caterpillars, bugs and borers (Fig. 1). These organisms can affect everything from the roots to the reproductive structures of plants. Some, such as aphids, also transmit viruses. To avoid damage and losses, integrated pest management should be adopted. This article specifically addresses aphids.

Aphids are historically the main pests of wheat and other winter cereals in Brazil. They cause direct damage through the suction and excretion of sap and damage to plant tissues, and are vectors of the "barley yellow dwarf virus" (BYDV), the causal agent of yellow dwarfism in wheat and other winter cereals.

With a high biotic potential, in a favorable environment, with the availability of host plants and the absence of natural enemies, aphid populations quickly reach alarming levels. Brazil's climate is beneficial to these insects, allowing them to maintain and expand their populations through the rapid process of asexual reproduction. Females produce nymphs, which go through four instars until they reach the adult phase, which will also be female, successively repeating the cycle. Adult forms can be wingless or winged.

In the 1960s and 1970s in southern Brazil, with the expansion of the Brazilian wheat area, the damage caused by aphids and transmitted viruses intensified. In a scenario of inefficient natural biological control, management was carried out with chemical insecticides. As these also affected natural enemies, a vicious cycle was created that intensified the imbalance in the agricultural ecosystem.

A historic milestone in wheat farming occurred in 1978, when, in order to reduce the damage caused by aphids, Embrapa Trigo, together with the Food and Agriculture Organization of the United Nations (FAO), launched the Wheat Aphid Biological Control Program (BCPPT). Through the BCPPT, 12 species of parasitoids were introduced and multiplied, totaling 3,8 million individuals released in the first five years, in the wheat-producing regions of southern Brazil. Some of these species established themselves, promoting a reduction in the population density of aphids. Currently, the populations of these pests are maintained in balance and chemical management mainly aims to reduce damage caused by virus transmission.

Aphid species

Several species of aphids occur at different times of the year and have a preference for certain phenological stages of the wheat plant. The most common species in wheat are described below.

• Green cereal aphid (Schizaphis graminum) - Apterous adults (Fig. 2A) have an elongated oval body with a general yellow-green coloration and a back with a dark green longitudinal midline; size ranging from 1,3 to 2,2 mm in length. It occurs in summer crops, such as corn and sorghum, and in winter crops, such as oats and wheat. Among the winter aphid species, it is the most prolific. Its biotic potential in association with the action of its saliva that triggers reactions in plant cells, causes this aphid to cause seedling death, mainly in wheat-growing regions where wheat is sown between February and April.

• Wheat stalk aphid (Rhopalosiphum padi) - Apterous adults (Figure 2B) have an oval body with a brownish-olive green color, with a reddish spot around and between the bases of the siphunculi. It is one of the most important species as a pest of wheat and other winter cereals in Brazil and worldwide. It has a wide host range, infesting various organs of the plant, from the roots to the reproductive structures. Its main importance is due to its efficiency in transmitting BYDV.

• Wheat ear aphid (Sitobion avenae) - wingless adults (Fig. 5C) with an elongated body, yellow-greenish, sometimes pinkish-beige, and black siphunculi. It is more abundant in the wheat earing phase and can cause quantitative and qualitative damage when in high populations during grain filling. Grain yield reductions of between 5% and 10% can occur. It transmits BYDV, however, as it colonizes the crop later in relation to R. padi, its damage as a virus vector tends to be less.

• Wheat leaf aphid (Metopolophium dirhodum) - wingless adults (Fig. 5D) with a fusiform body of pale green or light yellow coloration, a back with a darker longitudinal midline and measuring between 1,7 mm and 3,7 mm in length. It predominates at the time of year when wheat is elongating. In the 1970s, it was the main transmitter of BYDV in Brazil, but currently it loses importance to R. padi.

• Black cereal aphid (Sipha maydis) - Apterous adults (Fig. 5E) have an oval body with a general dark brown to black coloration, fully sclerotized, ranging in length from 1,0 mm to 1,9 mm and covered with thick bristles; small and inconspicuous siphunculi. This is the aphid associated with wheat that was most recently introduced into the Americas and Brazil. It forms very peculiar colonies at the base of the leaves. When feeding, it causes reactions in the plant cells and the colony area quickly turns yellow and becomes necrotic.

• Corn aphid (Rhopalosiphum maidis) - wingless adults (Fig. 5F) have an elongated yellow-green or blue-green body with black spots in the area around the siphunculi. It is common in corn and other summer grasses such as sudangrass (Sudanese sorghum). Larger than R. padi, is associated with higher temperatures, common in summer. It can occur in winter cereals, but in low frequency. It is not usually a problem for wheat crops, although it can transmit BYDV, such as BYDV-RMV, which possibly passes between winter cereals from corn through this vector.

Damage caused by aphids

Damage caused by aphids can result directly from feeding. Some aphids trigger rapid and severe reactions at the feeding site such as S. graminum (Figure 3A) and species of the genus Sipha (S. flava e S. maydis) (Figure 3B). Species that do not cause lesions at the feeding site cause significant direct damage if they reach high populations. This situation occurred with M. dirhodum in the pre-biological control period, with populations greater than 300 aphids per tiller. Currently, due to the efficiency of established biological control, this condition is not usually achieved. Direct damage estimated between 5 and 10% may occur for high infestations of the ear aphid.

The most common damage caused by aphids is due to the transmission of BYDV. For Brazilian wheat genetics, average grain yield losses of around 40% are common if virus infection occurs early in the cycle (first month after sowing). Comparing plots with and without insecticide treatment (seed treatment and aerial application), grain yield reductions of between 15 and 30% are usually obtained, varying with the year due to fluctuations in aphid populations. BYDV infection in wheat results in yellowing of the leaves (Figure 3C). Systemic symptoms are characterized by underdeveloped roots, shortening of internodes, reduced height, and delayed cycle (Figure 3D). Finally, the reproductive organs are also affected, with smaller ears, basal and apical sterility, fewer grains, and malformed and shriveled grains (Figures 3E and 3F).

Pest control

To reduce damage and obtain the best cost-benefit ratio, the various aphid control measures must be adopted in an integrated manner (table 1).

• Cultural control: Since aphid populations fluctuate throughout the seasons, there are times of greater risk. Planting in the middle of autumn occurs at a time when aphid populations are high and parasitoid populations are still low, a situation that is reversed in winter. Thus, the closer to winter, the lower the probability of BYDV transmission. On the other hand, for late plantings, the second annual peak of aphid populations should be monitored and control with insecticides implemented.

• Genetic control: Domestic wheat cultivars are susceptible to BYDV infection, but vary in tolerance. Intolerant materials can lose more than 60% of their productive potential, requiring greater protection through insecticides to prevent virus transmission.

• Chemical control: Seed treatment with neonicotinoid insecticides guarantees protection between 20 and 30 days after plant emergence. The initial phase of the crop is more vulnerable to BYDV infection and requires protection. Insecticide application to the aerial part is generally necessary after the effect of seed treatment has worn off. Depending on the region and sowing season, the number of applications will need to be varied. Monitoring of aphid populations is necessary for appropriate decision-making. The recommended action level is 10% of plants with aphids from the beginning of the crop until the end of booting. In ears, the action level is 10 aphids per ear.

• Biological control: reduces the growth potential of aphid populations by around 95%. The application of insecticides must consider the benefit of natural enemies so as not to cause disturbances that prevent this balance from being enjoyed. After their introduction and release in the late 1970s and throughout the 1980s, the parasitoids established themselves successfully, and no new releases are currently being carried out.

By José Roberto Salvadori (UPF and Embrapa Wheat); Alberto Luiz Marsaro Júnior (Embrapa Wheat); Crislaine Sartori Suzana-Milan (UPF); Douglas Lau (Embrapa Wheat); Eduardo Engel (Higher School of Agriculture "Luiz de Queiroz"); Mauricio Paulo Batistella Pasini (consultant); Paulo Roberto Valle da Silva Pereira (Embrapa Forests)

Article published in issue 288 of Cultivar Grandes Culturas Magazine