Evaluation of seed dressing insecticides for the control of maize lethal necrosis vectors

Abstract

Most of the overwhelming plant diseases caused by viruses including maize lethal necrosis (MLN) are attributed to viruses transmitted by vectors. The transmission of MLN from plant to plant by vectors provides the main means of spread in the field and that cause severe economic loss. Methods to control the vectors of plant virus diseases are intended at eliminating or altering one or more of the primary contributors (vector, virus, and host plant) in the transmission process or at preventing their coming together. The current experiments were conducted to evaluate seed-treatment insecticides for their efficacy on early season control of insect vectors of MLN causing viruses. The study on the effects of insecticide treatments on germination revealed that it does not significantly influence the germination of maize seed even up to 6 months storage before planting. Among the tested seed treatment insecticides, thiamethoxam 25% at 2.0 g/kg seed and imidacloprid + thiram at rate 1.5 showed superior control efficacy against maize thrips (Frankliniella sp.) with 96.06% and 95% population reduction, respectively, and maize leaf aphid (Rhopalosiphum maidis) with 97.37% and 96.67% reduction percentage, respectively. Hence, dressing of maize seeds before planting with such insecticides can be used for early-stage protection against potential vectors of the MLN causing viruses.

Keywords:

• Aphids
• insecticides
• seed dressing
• thrips
• vector

Reviewing Editor:

• Manuel Tejada, Universidad de Sevilla, Spain

Subjects:

• Agriculture & Environmental Sciences
• Agriculture
• Plant & Animal Ecology
• Entomology

Introduction

The emergence and rapid spread of plant virus disease can result in high epidemics and huge crop losses. Maize lethal necrosis (MLN) was emerged as a serious threat to maize production in eastern Africa in 2011 and since then the disease is causing from low to complete loss of maize production in the region (Mahuku et al., 2015b; Wangai et al., 2012). MLN is a viral disease caused by double infection of Maize chlorotic mottle virus (MCMV) with any one of potyviruses, namely Maize dwarf mosaic virus (MDMV), Wheat streak mosaic virus (WSMV), Sugarcane mosaic virus (SCMV), or the recently described Johnsongrass mosaic virus (JGMV) (Stewart et al., 2014, 2017).

The outbreak of MLN caused by co-infection of MCMV and SCMV in Ethiopia has been first observed and reported in the Upper Awash Valley of Oromia region in 2014 (Mahuku et al., 2015a). The disease has since spread to other major maize producing regions of the country including Oromia, South Nation, Nationality and Peoples (SNNP), Amhara, Benishangul-Gumuz and Tigray (Bekele et al., 2017; Fentahun et al., 2017; Guadie et al., 2019; Regassa et al., 2020). MLN disease infects maize plants at different growth stages starting from early to near-maturity stage of the crop. Early infections lead to complete plant death while late infections lead to plants aging prematurely, male sterility, and malformed or no ear or production of deformed seed (Regassa et al., 2021; Uyemoto et al., 1981; Wangai et al., 2012).

MLN causing viruses have insect vectors that spread into, between, and within crops. Maize thrips, Franklinella sp. (Thysanoptera: Thripidae) and cereal leaf beetle, Oulema sp. (Coleoptera: Chrysomelidae) transmitted MCMV, whereas corn leaf aphids, Rhopalosiphum maidis Fich (Hemiptera: Aphididae) transmitted SCMV (Jiang et al., 1992; Cabanas et al., 2013; Regassa et al., unpublished data). The disease is also transmitted through virus infected seed at low rate (Jensen et al., 1991; Regassa et al., 2021) and contaminated soil and infected maize residue (Mekureyaw, 2017; Regassa et al., 2022).

Control measures against vectors and vector activities can include reducing vector populations, reducing virus sources, interference with vector landing, and interference with the transmission process (Fereres & Raccah, 2015). The effective management of MLN causing vectors is vital to minimizing the spread of MLN disease, consequently reducing maize yield loss. Insecticide seed dressing/treatments are applied to seeds to control insect pests. Plant viruses including MLN cannot be directly controlled by the use of pesticides, however, certain systemic insecticides can control the insect vectors that carry viruses from host to host. Several insecticides, formulated either as granules or spray applications can be used to manage vectors.

Seed dressing in agriculture involves the treatment of seeds with insecticides and/or fungicides in order to fight above-and belowground insects and soil-borne fungal diseases (Taylor et al., 2001). Before planting, application of seed treatment using systemic insecticides can provide early-stage protection against thrips, aphids, and other potential vectors of the MLN including beetles (Alford, 2000). Thiamethoxam and imidacloprid singly or mixed with other pesticides are commonly used as a systemic seed treatment to protect seeds and seedlings against injury by early season insects (Tharp et al., 2000; Wilde, 1997; Wilde et al., 2001). Both imidacloprid and thiamethoxam have the potential to provide long-term residual control of a broad spectrum of insect pests (Maienfisch et al., 2001; Wilde et al., 2001). However, available and registered seed dressing insecticides were not evaluated against insect vectors of MLN causing viruses in Ethiopia. Therefore, the objective of this study was to evaluate seed-treatment insecticides for their efficacy on early season insect vectors of MLN causing viruses.

Materials and methods

The experiment was conducted under greenhouse condition at Ambo Agricultural Research Center, in Oromia, Ethiopia. The center is located at 8^º58′10″N latitude and 37°51′28″E longitudes and at an altitude of 2164 m above sea level. The area has a warm humid climate with mean monthly minimum, maximum temperatures, and average total annual rainfall of about 12.07 °C, 26.13 °C, and 1068 mm, respectively.

Corn leaf aphid (Rhopalosiphum maidis) adults used in this study had been identified as vector of SCMV, and maize thrips (Frankliniella sp.) identified as vector for MCMV (Regassa et al., 2024).

Plant materials and seed treatment

MLN susceptible maize seed (BH661) obtained from Bako Agricultural Research Center was used for the study. Four registered systemic seed dressing insecticides in Ethiopia including Apron star 42% WS (thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2%), Evident 25% WG (Thiamethoxam 25% WG), Imidalm T 450 WS (imidacloprid + thram), and Proseed Plus 63% WS (Imidacloprid + Thiram + Carboxin) were used.

The seeds were dressed uniformly with three rates of application for each treatment (recommended, lower, and above respective dosage of insecticides; Table 1). The solution volume used (product + water) was 8, 10, 7, and 10 ml per 1 kg of seeds for Apron star 42% WS, Thiamethoxam 25% WG, Imidalm T 450 WS, and Evident 25% WG, respectively. The product slurry was distributed over 1 kg of seeds with respective dosage/application rate of insecticide in the bowls and stirred for 5–10 min to coat seed uniformly with the in: 50–60%; temperature: 20–25 °C). The dried seeds treated with respective dosage of insecticides were packed in separate polythene bags and kept under laboratory condition until sowing. After 1 and 6 months storage, its effect on seed germination and vector management were evaluated. Undressed seeds were used as a control.

Table 1. Descriptions of insecticides (treatments) used for seed treatment for control of maize lethal necrosis transmitting insect vectors and rates of application.

Trade name	Active ingredient	Treatment with code	Application rate (g/kg seed)
Apron Star 42% WS	thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2%	T1 = Apron star 42% WS	2.5
		T2 = Apron star 42% WS	2.0
		T3 = Apron star 42 % WS	3.0
Imidalm T 450 WS	imidacloprid + thiram	T4 = Imidalm T 450 WS	1.0
		T5 = Imidalm T 450 WS	0.5
		T6 = Imidalm T 450 WS	1.5
Proseed Plus 63% WS	Imidacloprid + Thiram + Carboxin	T7 = Imidacloprid + Thiram + Carboxin	3.0
		T8 = Imidacloprid + Thiram + Carboxin	2.0
		T9 = Imidacloprid + Thiram + Carboxin	1.0
Evident 25% WG	Thiamethoxam	T10 = Thiamethoxam 25% WG	2.0
		T11 = Thiamethoxam 25% WG	1.0
		T12 = Thiamethoxam 25% WG	3.0
		T13 = Undressed seeds used as control	–

Determination of seed dressing insecticides on germination

The effect of insecticide seed treatments on the germination rate and storage time was evaluated two times after 1 and 6 months storage. Randomly selected 20 seeds from each treatment of the stored seed at one and six months were placed on water moist double layer of filter paper. Water was added into each plate as required to keep the filter paper moist. The experiment was conducted in Completely Randomized Design (CRD) with three replications. The test was conducted in greenhouse at temperature of 25–30 °C during day and 18 °C at night. Following standard seed germination determination by Gorim and Asch (2012), a seed with visible radicle (longer than 2 mm) was considered as germination. Number of newly germinating seeds was recorded daily for 8 consecutive days, and the cumulative germination rate was calculated.

Seed germination was recorded daily up to 8 consecutive days, after the start of the experiment. Germination percentage (GP) was calculated according to the International Seed Testing Association (ISTA) method $$\text{GP}=\left(\frac{\text{Number\hspace{0.33em}of\hspace{0.33em}normally\hspace{0.33em}germinated\hspace{0.33em}seeds}}{\text{Total\hspace{0.33em}number\hspace{0.33em}of\hspace{0.33em}seedssown}}\right)\times 100$$

Insect rearing

Adults of corn leaf aphid (Rhopalosiphum maidis) and maize thrips (Frankliniella sp.) used in this study had been originally collected for MLN causing viruses vector transmission study (Regassa et al., 2024) and maintained in a greenhouse on maize plants. Maize seedlings were grown in 25 cm diameter plastic pots filled with a sterilized soil mixture (soil, sand, and yard manure in the ratio of 2:1:1). Initially individual species were separately established by transferring adult insects to healthy maize seedlings and placed in insect-proof cages with a photoperiod of 12 h (light/dark) and a temperature range of 25–30 °C. Adult insects were allowed to lay eggs for 1 week and then removed from cages and the eggs laid were permitted to develop into adults, which appeared in approximately 2–3 weeks.

Determination of effect of seed treatment on insect vectors

The effect of different seed treatments on vectors of MCMV (Frankliniella sp.) and SCMV (R. maidis) were assessed in an insect proof cage at room temperature ranging from 25–35 °C. Treated seeds were planted in steam-sterilized soil mixture of soil, sand, and organic manure at 2:1:1 ratio, respectively inside 25 cm diameter plastic pots (five seed per pot). After plants were germinated and reached two leaf stages, each colony of vectors from rearing cage were introduced/transferred (20 Frankliniella sp. and 30 R. maidis per plant) on to the maize seedlings developed from seeds treated with insecticides. Adult and immature thrips were exposed to seedlings developed from treated seeds. Three pots (replications: five plants per pot) were established for each application rate of each insecticide. Treatments were evaluated by counting the number of live vectors staring from the third day after insects transferred to maize seedlings at 7 days intervals for 2 consecutive weeks (i.e. at 3, 10, and 17 days).

Data analysis

The percentage of reduction (% R) of the insect population was calculated according to the following equation (El-Naggar & Zidan 2013).

$\% R=\left(\frac{\mathit{\text{NIC}}-\mathit{\text{NIT}}}{\mathit{\text{NIC}}}\right)x 100$, where NIC = number of insects in the control and NIT = number of insects in the treatment.

Statistical analyses were carried out using the SAS procedure of GLM (SAS Institute, Cary, NC, USA). Least significant differences were calculated with an analysis of variance using the LS means statement in the general linear model procedure of SAS software (SAS Institute 9.4 Cary, NC, USA).

Results

Effect of insecticidal seed treatments on maize seed germination

The germination percentage of maize seed after being treated with four systemic insecticides at different rates is presented in Table 2. The results showed that there were no significant differences among the treatments. The insecticides did not affect the germination percentage even up to 6 months of storage after treatments. However, germination was higher (100%) for Apron star 42% WS @2 g/kg, Imidalm T 450 WS @ 1 g/kg, Imidacloprid + Thiram + carboxin @2 g/kg, Apron star 42% WS @ 2.5 g/kg treated seeds and untreated seeds, and lower for Thiamethoxam 25% WG (95–96.67%). Irrespective of insecticides used in seed treatment the germination percentage did not differ significantly over different periods of storage. As compared to the undressed control, germination slightly decreased from 100 to 96.67% in treated seed stored for 1 month and 98.33 to 95% for treated seed stored for 6 months before planting (Table 2). These levels of reduced germination were comparatively small, never exceeding 4% of the controls in the final evaluation. Comparing all tests, the germination percentage ranged from 95 to 100%.

Table 2. Influence of seed treatment on the germination (%) of maize seeds stored for 1 and 6 months after insecticide treatment.

Treatment	Rate (product in g per 1 kg seed)	Germination (%)
		After 1-month storage	After 6-month storage
thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2%	2.5	100.00a	100.00a
	2.0	100.00a	98.33ab
	3.0	98.33aa	98.33ab
Imidacloprid + thiram	1.0	100.00a	98.33ab
	0.5	98.33a	98.33ab
	1.5	98.33a	98.33ab
Thiamethoxam 25% WG	2.0	98.33a	95.00b
	1.0	96.67a	96.67ab
	3.0	96.67a	95.00b
Imidacloprid + Thiram + Carboxin	2.0	100.00a	98.33ab
	1.0	98.33a	98.33ab
	3.0	98.33a	98.33ab
Undressed seed	0	100.00a	98.33ab

Means followed by the same letters are not significantly different (p < 0.05).

Effect of insecticidal seed treatments on control efficacy against thrips

The population of the thrips (Frankliniella sp.) were significantly lower on maize seedling developed from the treated seeds than those seedlings developed from the undressed control (F12, 38.22 = 76.37, p < 0.0001). All the insecticidal treatments reduced Frankliniella sp. population over undressed control and the reduction varied from 90.67 to 99.67% for treated seeds stored for 1 month and 60.00 to 95.33% for treated seeds stored for 6 months before planting.

In these seed-treatment tests against MCMV vector (Frankliniella sp.), all the tested dosages of imidacloprid + thiram were more effective in reducing Frankliniella sp. than the other treatments and the untreated control in the treated seeds stored for 1 month. Whereas, thiamethoxam 25% at 2.0 g/kg and imidacloprid + thiram at the rate of 1.5 showed superior control efficacy against Frankliniella sp. on both maize seedlings derived from treated seeds stored for 1 and 6 months before planting (Table 3).

Table 3. Effects of insecticidal maize seed treatment on thrips (Frankliniella sp.) population reduction in maize seedlings raised from treated seeds stored for 1 and 6 months.

Treatment	Seedlings derived from treated seeds stored for 1 month	Seedlings derived from treated seeds stored for 6 months
Thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2%@2.5 g/kg	95.78e	81.78c
Thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2%@2 g/kg	97.33bcd	84.11bc
Thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2% @3 g/kg	96.22cde	82.44c
Imidacloprid + thiram @1 g/kg	98.89a	85bc
Imidacloprid + thiram @0.5 g/kg	99.22a	82.44c
Imidacloprid + thiram @1.5 g/kg	99.11a	90.89ab
Imidacloprid + Thiram + Carboxin@3g/kg	97.22bcd	81.78c
Imidacloprid + Thiram + Carboxin@2g/kg	96.00de	73.56d
Imidacloprid + Thiram + Carboxin@1g/kg	91.11f	69.89d
Thiamethoxam 25% WG@2g/kg	98.33ab	93.78a
Thiamethoxam 25% WG@1g/kg	96.00de	91ab
Thiamethoxam 25% WG@3g/kg	97.44bc	98abc
Undressed seed	3.33g	2.44e

Means with the same letter are not significantly different (p ≤ 0.05).

Effect of seed treatments on control efficacy against R. maidis

The effectiveness of the seed treatments showed that aphid number on maize seedlings decreased from 99.48–96.15% in seedlings derived from treated seeds after 1 month of storage after treatment and 80.59–95.85% in seedlings derived from treated seeds after 6 months of storage after treatment per application rate. This data indicated that R. maidis population reduction in the treated over untreated control ranged from 80.5 to 99.48%, and seed treatment was found most effective against R. maidis when thiamethoxam 25% WG applied @2 g/kg and Imidalm T 450 WS @ 1.5 g/kg were used (Table 4).

Table 4. Effect of insecticidal maize seed treatment on maize leaf aphid (Rhopalosiphum maidis) population in maize seedlings derived from seeds stored for 1 and 6 months after treatment.

Treatment	Seedlings derived from treated seeds stored for 1 month	Seedlings derived from treated seeds stored for 6 months
Thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2%@2.5 g/kg	97.19dc	87.85abc
Thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2%@2 g/kg	98.22abc	90.15a
Thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2% @3 g/kg	97.48dc	88.3abc
Imidacloprid + thiram @1 g/kg	99.26ab	90.00a
Imidacloprid + thiram @0.5 g/kg	99.48a	88.30abc
Imidacloprid + thiram @1.5 g/kg	99.41a	93.93a
Imidacloprid + Thiram + Carboxin@3g/kg	97.78bc	88.89ab
Imidacloprid + Thiram + Carboxin@2g/kg	98abc	80.59c
Imidacloprid + Thiram + Carboxin@1g/kg	96.15d	81.03bc
Thiamethoxam 25% WG@2g/kg	98.15abc	95.85a
Thiamethoxam 25% WG@1g/kg	97.85bc	91.78a
Thiamethoxam 25% WG@3g/kg	98.52abc	93.41a
Undressed seed	2.08e	1.63d

Means with the same letter are not significantly different (p ≤ 0.05).

As that of Frankliniella sp., imidacloprid + thiram at all dosage and storage time were effective and showed insecticidal activity against R. maidis., while imidacloprid + thiram + carboxin at 1.0 and 2.0 g/kg (lower and medium dosage) were showed lower reduction percentage on both maize seedlings developed from treated seeds stored for 1 and 6 months before planting (Table 4).

Cumulative effect of different seed dressers on MLN causing virus vectors

The cumulative effect of different seed treatments on Frankliniella sp. (vector of MCMV) and R. maidis (vector of SCMV) population exposed to maize seedlings developed from treated seeds stored for up to 6 months is given in (Table 5). Significant reduction of both vector species population was indicated in all treatments over the undressed control. The results revealed that the treatments Evident 25% WG (thiamethoxam 25% at 2.0 g/kg seed) and imidalm T 450 (imidacloprid + thiram) at rate of 1.5 were significantly superior in reducing of both species population when compared with the others. While Proseed Plus 63% WS (imidacloprid + thiram + carboxin) @1 g/kg is the lowest in reduction of Frankliniella sp. and R. maidis population (Table 5).

Table 5. Cumulative effects of different insecticidal seed dressers on MCMV and SCMV vectors, thrip (Frankliniella sp.). and (Rhopalosiphum maidis), respectively.

Treatment	Rate (g/kg maize seed)	Mean population reduction of thrips (Frankliniella sp.)	Mean population reduction of aphid (R. maidis)
Thiamethoxam 25% WG	2.0	96.06a	97.37a
Imidacloprid + thiram	1.5	95.00ab	96.67ab
Thiamethoxam 25% WG	1.0	93.50bc	94.56abc
Thiamethoxam 25% WG	3.0	93.22bc	95.85abc
Imidacloprid + thiram	1.0	91.95dc	94.63abc
Imidacloprid + Thiram + Carboxin	3.0	91.50de	93.33bc
Imidacloprid + thiram	0.5	90.83def	93.89bc
thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2%	2.0	90.72def	94.18abc
thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2%	3.0	89.89ef	92.89bcd
thiamethoxam 20% + metalaxyl – 20% + difenoconazole 2%	2.5	89.72f	92.52 cd
Imidacloprid + Thiram + Carboxin	2.0	84.78g	89.30de
Imidacloprid + Thiram + Carboxin	1.0	77.72h	87.77e
Undressed seed		2.89i	1.85f

Means are from both two treated seeds stored for 1 and 6 months. Means followed by the same letters are not significantly different (p < 0.05).

Among the different dosages of seed treatments tested against Frankliniella sp., thiamethoxam 25% WG at the rate of 2.0 g/kg seed was superior over thiamethoxam 25% WG with the rates of 1.0 and 3.0 g/kg seed. Among dosages of imidalm T 450 WG (imidacloprid + thiram), imidalm T 450 WG @ 1.5 was significantly superior than imidalm T 450 WG @ 0.5 and 1.0 g/kg seed (Table 5).

Discussion

Maize is attacked by various sucking insect pests and chewing species during the growing season. Furthermore, most of these insects can carry and spread viral diseases among plants via feeding from infected plant to healthy one. Plant virus diseases including MLN causing viruses (MCMV and SCMV) can be controlled to some level by controlling the vectors that transmits the virus. In Ethiopia, maize thrips (Frankliniella sp.) and corn leaf aphid (Rhopalosiphum maidis) are the major vectors of MCMV and SCMV, respectively (Regassa et al. unpublished data). The vectors, especially, thrips breed quickly at high temperatures and continually migrate to newly emerged maize leaves and are also too small to be easily identified and are usually not directly exposed to foliar sprays, as they are mostly concentrated on internal leaves (Ding et al., 2018). As compared with foliar sprays, systemic seed treatments provide a good solution for such problem because the strong upward passage allows insecticides on seeds to be continuously absorbed and transferred to new leaves throughout the seedling level (Alford and Krupke, 2017; Elbert et al., 2008).

Seed dressing with systemic insecticide is an important part of pest management tactics, which is a relatively less pollutant to the environment, (Nault et al., 2004; Taylor et al., 2001). Insecticidal seed treatments against MLN disease insect vectors may be an essential control measure in the early maize growth stages to delay the rate of transmission of both MCMV and SCMV or their infecting load injected by the vector into the maize plants.

In the current study, insecticide seed treatment did not significantly influence the germination of maize seed even up to 6 months storage before planting. Accelerated germination in thiamethoxam-treated maize (Horii et al., 2007), and in soybean (Cataneo et al. 2010) was reported. The slight decline in germination percentage may be due to aging effect leading to depletion of food reserves (Laxman et al., 2017).

The current study conducted on the vectors of MLN causing viruses indicated that the control efficacy differed among seed dressing insecticides with different dose. More satisfactory reduction levels of maize thrips (Frankliniella sp.) and corn leaf aphids (Rhopalosiphum maidis) were achieved using thiamethoxam 25% WG at the rate of 2.0 g/kg seed than other insecticides used in this study at the same dose. In similar of this study, Ding et al. (2018) demonstrated that treating maize seeds with thiamethoxam (1.0 and 2.0 g/kg of seeds) reduced thrips infestations on maize under field conditions. However, compared with other tested seed dressing insecticides, Imidacloprid + Thiram + Carboxin (1.0 g/kg of seeds) had a lower control effect for both vectors. The differences in efficacy might be associated with the toxicity of the different insecticides to thrips. As reported by Byrne et al. (2007) other than maize plant, thiamethoxam and imidacloprid provide good control of avocado thrips in bioassays. The toxicities of thiamethoxam to larvae and adult females of western flower thrips (Frankliniella occidentalis) were higher than those of other tested neonicotinoids nitenpyram, imidacloprid, and thiacloprid (Shan et al., 2012).

The findings from the current study indicate that insecticide seed dressing of maize seeds before planting with thiamethoxam 25% WG at the rate of 2.0 g/kg seed or Imidalm T 450 at the rate of 1.5 g/kg of seeds can be used for early-stage protection against potential vectors of the MLN causing viruses in Ethiopia. Furthermore, evaluation of these insecticides in the field will be suggested against MLN causing viruses insect vectors infestation and virus transmission provides a real test of its effectiveness.

Contributions

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by (Bayissa Regassa and Adane Abraham). The first draft of the manuscript was written by Bayissa Regassa and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Additional information

Funding

This research was financially supported by the then Ethiopian Ministry of Science and Technology (now Ministry of Innovation and Technology), and the Ethiopian Institute of Agricultural Research.