Corresponding authors: Attila-Károly Szabó (
Academic editor: J. Sun
We conducted field surveys to detect the population density of the most important invasive weed species and their associated virus vectoring aphids in crops grown under high input field (HIF) vs low-input field (LIF) conditions, with and without fertilizers and pesticides. The most frequent invasive weed species were
Szabó A-K, Kiss J, Bálint J, Kőszeghi S, Loxdale HD, Balog A (2019) Low and high input agricultural fields have different effects on pest aphid abundance via different invasive alien weed species. NeoBiota 43: 27–45.
Invasive pests represent serious threats to crop production as global trade expands and climatic conditions shift (
The aim of the present study was to: a) assess the population density of the most important invasive weed species when agricultural crops are managed with high-input fertilizers and chemical pesticides (high-input fields,
Experiments were conducted during the crop growing (vegetative) seasons of 2015 and the 2016 in Central and Eastern Transylvania, Romania in order to assess the population density of the most important invasive weed species and infesting virus-vectoring aphids, both from low- and high-input agricultural crops.
Fertilizer and pesticide input on crops under high intensity management (HIF) in the two study years.
Crop | Treatments | |
---|---|---|
Potato | Fertilizer | N, P, K (15,15,15) 0.2 t/ha |
Herbicide | Sencore (metribuzin70%) | |
Titus 25 DF (rimsulphuron) | ||
Insecticide | Calypso (tiacloprid) | |
Fungicide | Banjo (fluazinam) | |
Ridomil Gold (mefenoxam, mankoceb) | ||
Infinito (62.5 g/l fluopicolide + 625 g/l propamocarb clorhidrat) | ||
Consento (375 g/l propamocarb clorhidrat + 75 g/l fenamidon) | ||
Acrobat Mz (difenomorf, mankoceb) | ||
Alfalfa | Fertilizer | N, P, K (15,15,15) 0.16 t/ha |
Herbicide | Pallas (piroksulam) | |
Insecticide | Fastac (alfa-cipermetrin) | |
Falcon Pro (protioconazol 53 g/l + spiroxamină 224 g/l + tebuconazol 148 g/l) | ||
Fungicide | Amistar Xtra (azoxistrobin) | |
Maize | Fertilizer | N, P, K-15,15,15 0.15 t/ha |
Herbicide | Adengo (isoxaflutol 225 g/l + tiencarbazon-metil 90 g/l + ciprosulfamide (safener) 150 g/l) |
The studied fields from the two regions were situated in the same altitudinal range of about 250 m a.s.l. and under comparable bioclimatic conditions. The distance between the studied areas was about 200 km.
Three weed and two native aphid species were studied, these being the most common species in the study area. The most important weed species, all of the family
The two native aphid species included in this study where the highly polyphagous leaf-curling plum aphid,
First, we selected two study sites in each region, these being 10 km in a fist and 15 km distant in a second region from each other. In each site we established two transects (at least at 1 km apart) of 10 m long × 1 m wide at an approximately equal distance of at least three major crops (maize, potato and alfalfa). In this way, each transect was surrounded by at least 8–10 ha of high-input, and at least 0.5–3 ha area of low-input, agricultural crops. Each transect was carefully measured and located using GPS. Second, inside each transect we placed ten 1 m2 quadrats. Each of these quadrats was further subdivided in 10 × 10 cm plots (100 for each quadrat) and all plants (native and invasive) were counted and their coverage estimated within them. (
Because plants contained aphid colonies, and the exact number of individual aphids was important, all bags were labelled and kept at low temperature (near 0–4 °C in a cool box), then returned to the laboratory, whereupon all samples were stored at −20 °C, and aphids counted and species identified (
Leaf samples used for enzyme analyses were collected each year from each abundant weed species per experimental field, sub-area and transect, starting from the first until the last assessment. Separate young leaves, all containing aphids, were collected from the weeds (
For extraction, 400 mg of frozen leaves were homogenized in 1 ml of 50 mM phosphate buffer, pH 7.0, using a FastPrep Instrument high-speed benchtop homogenizer (MP Biomedicals). The homogenate was centrifuged at 6,500 r.p.m. for 20 minutes at 4 °C, and the supernatant collected. Protein concentration of the enzyme extract was determined by the Bradford method (
The experiment was performed during the vegetative period of 2017 by setting-up 30 blocks of the two most abundant weed species,
Aphid colonization experiment design, weed plants of
After acclimatization in May, similar size plants of about 30 cm were selected for experiments. Crop plants of maize, potato and alfalfa were also cultivated in 8 litre pots, and similar sized plants selected in May for experiments. All weed and crop plants were first cleared of any infesting aphids by visual checking of all leaves and shoots. In the case of any aphid colony being detected, these were removed by brushing off colonies from the plants with a soft paint brush. If other insect species were detected, these were also removed. Insect-cleared plants were then allocated for experiments. Altogether 30 experimental blocks were set-up, 15 blocks with
The established aphid colonies (assessed by careful visual assessment over a 10 minute period as to whether aphids were feeding consistently on plants and not moving) were checked after 24, 48 and 72 hours. If no colony establishments were detected, new colonies were placed on the weeds. The aphids were then left to reproduce for 10 days. The assessment of aphids started after 14 days after aphid colony establishment, such that enough winged individuals were present to leave weeds and colonize crop plants. Aphid numbers were assessed on both weed and crop plants of the same blocks starting from mid-May as follows: two randomly selected blocks (one with
On return to the laboratory, the entire content was stored at −20 °C and the next day all samples were carefully assessed for aphids and their respective numbers counted under a stereo microscope. By this means, all individual aphids were captured and counted. The same procedure was repeated the following day, until the total number of blocks and plants were sampled by cutting all and aphids from plants counted in lab. The entire sampling was done within a two-week period and completed by the end of May, a time when aphid migration to new host plants occurs. All aphids were counted, recorded regarding the weed and crop plants they were collected from, and identified to species level.
For weed data, the mean coverage in each 1 m2 quadrats was determined by averaging the plant values from 10 × 10 cm plot. Next the inter-annual differences in coverage were tested using multivariate analysis of variance (
All aphid species were correlated with particular weed species. In the case of one individual weed plant hosting two aphid species, the percentage of the species were considered. This was the case in only 7% of all the samples examined. It was then determined how cropping system differentially affected associational susceptibility to the two aphid species,
POD activity values obtained were compared between years (MANOVA) considering values from 1 m2 sub-transects per sampling period. No significant year effect was detected (
Effects of weed plants on
Three invasive weed species were dominant during the two years field assessment.
The most frequent invasive weed average coverage between management systems. LIF = low-input field, HIF = high-input field. Data were compared using Kruskal-Wallis test, followed by Mann-Whitney U test.
Management | Weed species | Aver. Cov.(%) | median | 25th/75th quart. | U |
|
---|---|---|---|---|---|---|
LIF |
|
97.50% | 98.5 | 95/99 | 2.19 | 0.02 |
|
2.50% | 1.5 | 1/5 | |||
HIF |
|
84.50% | 84.5 | 83/86 | 2.16 | 0.03 |
|
15% | 14.5 | 13/16 |
The two important aphid species were detected in high densities on all three dominant invasive weeds. The most frequent species was
The average
No observable differences in POD enzyme activity were detected for
Linear correlation between POD activity level at 5 and 10 μmol min−1 · mg protein−1 unit and aphids (
Correlation | POD 5 μmol | POD 10 μmol | ||||||
---|---|---|---|---|---|---|---|---|
|
|
|
|
|||||
r |
|
r |
|
R |
|
r |
|
|
LIF | −0.67 | 0.21 | −0.74 | 0.14 | −0.76 | 0.12 | −0.72 | 0.16 |
HIF | 0.42 | 0.46 | 0.34 | 0.56 | −0.48 | 0.4 | −0.3 | 0.61 |
The number of
Colonization rate of
Comparison between colonization rate of
Here we showed that associational susceptibility can be detected between the most frequent weed and crop plants under the different crop management regimes. The high invasive weed density harbours a concomitantly higher aphid population density comprising local species. More precisely, a 13% higher coverage difference of
The colonization experiment also revealed that
No clear associational susceptibility was however detected when comparing POD enzyme activity on
The idea that plant-induced POD activity increases as a consequence of sap-feeding insect activity was first suggested by
The relevance of our study is threefold: environmental, crop management, as well as aphid control. In terms of environmental management, although low-input management farming systems are widely studied (
From a crop management perspective, new management systems and new assessment methods are necessary to evaluate the possible effect of weeds on vegetable and cereal crops due to aphid activity, both physical (i.e. direct feeding damage) and more importantly, via transmission of one or more plant pathogenic viruses.
Lastly, from the standpoint of aphid control and associated virus transmission, the complete lack of any management needs to be reconsidered. This is because high aphid density and possible virus infestation can make the cultivation of some crops under low-input systems difficult, if not impossible. These crops (potato) are, however, considered low-cost and low-input crops, and hence are widely cultivated under low-input management regimes. From our study, it is clear that cultivation methods, including invasive weed control, need to be synchronised and vector controls reconsidered, even if no other management is planned.
The authors are grateful to all those, especially local people, who made our assessments possible. We would especially like to thank Rajmund Simpf and Zsófia Simó for assisting in the initial stages of weed and aphid assessment. This research was supported by the Higher Education Institutional Excellence Program (1783-3/2018/FEKUTSTRAT) awarded by the Ministry of Human Capacities within the framework of plant breeding and plant protection researches of Szent István University.