Background

The thread caterpillar Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae) is an insect pest that attacks around 30 important crops such as broccoli, lettuce, carrots, potatoes, beans and other crops [1]. This insect pest when in the fourth instar feeds on the leaves and in the early stages of development there is little loss of foliage, but at the end of the fourth stage they can cut many young plants in a single night [2].

The use of synthetic insecticides is effective to control the population of A. ípsilion, but in recent years it has been observed Pest´s resistance to some insecticides, in addition to the negative implications that come with constant use of insecticides ranging from water pollution and food contamination that can affect human health and the imbalance of the ecosystem [2]. Therefore, a pest control strategy that doesn’t attack environment has been studied. The intercropping is an important cultural practice in pest management as this is to diversify crops in a given agro-ecosystem to reduce the population of insects and consequently their attack [3, 4]. Studies indicate that intercropping protects the target plants using various mechanisms. Non-host plants involved in the mixture can emit volatile organic compounds which adversely affect the pest insects, giving thus some level of protection. This level of protection may involve the attraction of natural enemies of the pest insect or acts repelling the insect pest. For example, Khan et al. [5] observed that the intercropping with non-host plants such as molasses grass corn has significantly decreased the levels of infestation by stem borers and also increased larval parasitism of drills by Cotesia sesamiae. Volatile compounds produced by non-host plants repelled the insect pest, drill logs and even attracted the female parasitoid C. sesamiea. Landolt et al. [6] tested 27 essential oil extracts from 27 plants as a protective barrier against moth in fruit, and these consisted that the most effective repellents are garlic, Allium sativum L, Pogostemom cablin (Blanco) and Tanacetum vulgare L. These results suggest that the plant or plant oils may be used as a barrier against the fruit moth.

Moreover, the simultaneous emission of volatile compounds from two or more crops in the field hinders the location host plant. Another way is the possibility of plant–plant communication where the plant can host not signal via the volatile compounds and this can protect themselves from future attacks. The other possibility is to use a plant as a sacrifice, i.e., the main mix with a plant culture, which is easily attacked by the insect pest [7].

Song et al. [8], showed that intercropping of apples orchids with three different aromatic plants ageratum (Agerarum houstonianum Mill.), French marigold (Tagetes patula L.) and basil (Ocimum basilicum L) enhanced a biological control of Aphis citricola Van der Goot. We believe that these results are related with volatiles organics compounds released by these aromatics plants.

Although there are many studies that use intercropping as a way to control insect pests but there are necessary more researches because depending on the agro-ecological, genetic variability and meteorological conditions the results may be different. So this study aims to evaluate the effect of onion Allium cepa L, Red Creno landrace in controlling thread caterpillar, dark sword-grass in lettuce by intercropping with onion in agro-ecological conditions of Lichinga.

Methods

Insects

The work was performed in Lichinga, capital town of Niassa Province, Mozambique (Fig. 1). A field was prepared in Muchenga neighborhood (where the caterpillars were identified) with a total of three plots for growing lettuce that allowed the development of caterpillars screw. This activity was carried out in this neighborhood because they already knew the basic conditions caterpillar occurrence of that neighborhood such as moisture, soil type and the type of lettuce that most appeals to A. ipsilon which is the lettuce crisp. The cultivation of lettuce was taken under natural conditions without the presence of agents that could repel and/or kill A. ipsilon.

Fig. 1
figure 1

Location of study area

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At the time that the larvae were identified and were removed from mechanically ground and then introduced into a wooden box with similar soil in place of rearing conditions and thus transported to the test location where they were introduced in two portions.

It is important to mention that the caterpillars of A. ipsilon appeared in themselves creating the field, it was not necessary to lay eggs, or adult moths or larvae during this period of creation caterpillars fed on lettuce until the time of their removal.

Plants

They were prepared in two seedbeds where one contained a variety of onion (Red Creno) and the other contained a variety of lettuce (crisp head lettuce). The onion seed was conducted two weeks before the lettuce seed to coincide with the development of the two cultures, although knowledge of the onion is presenting very long life cycle as compared to lettuce.

In seedbeds, the seeds were laid in rows, and a week after germination in seedbeds of both cultures were clean (mulching) so as to eliminate the possible weeds present in them.

Seedlings of both crops were transplanted into the final installments on the same day, in the morning period this activity was carried out 3 weeks after the emergence of lettuce. Furthermore, the test site were prepared six portions distributed in two portions, the first portion was intercropped and the second was a control which had only lettuce plants.

Intercropping assay

For intercropping crop the aid was necessary from a tape-metric for measuring portions and compasses to use on these crops. The bars that were used varied with the culture; thus, for lettuce crop was used a measure 30 cm from one plant to another and from one line to another; for onion crop was used a compass 10 cm distance between a plant of another within the line.

Thus, three different forms of the consortium were used, in which the first has alternating cultures of lines (one line onions and other lettuce), the second was the toggle also cultures of lines (two lines of lettuce and onion); finally, the third way was to put a solid row of onions around the plot and lettuce plants were placed in the middle of the plot (Fig. 2).

Fig. 2
figure 2

Intercropping patterns: A- onion continuous line around a line of lettuce- Surround; B- a line of onion and one lettuce line C-two rows of lettuce and one onion line

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Data collection and statistical analyses

At the end of each test, the number of intact lettuce plants were counted and attacked at the field in intercropping and control field (with only lettuce), and in the different intercropping patterns. The mean comparison was based on ANOVA followed by Tukey’s test.

Results

According to the experiments, it was observed that the control field compared to the field in intercropping showed a fewer number of lettuce plants not attacked (P < 0.05) by the caterpillar after the test. This result can be seen in the chart below (Fig. 3).

Fig. 3
figure 3

Average Percentage of lettuce not attacked after the test in monocrop (only lettuce) and intercropping (lettuce and onion) (N = 5; p < 0.05). (Mean values ± SE followed by different letters are significantly different (ANOVA, LSD test, differences considered significant at P < 0.05)

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In this experiment we also tested three patterns of intercropping, namely: continuous onion queue around the hotbed (Surround), switching crops between rows (a row of onion and other lettuce) and switching crops between rows (two lines lettuce and onion). The intercropping in Surround had significant a higher average (p < 0.05) number of plants not attacked after testing compared with other forms of intercropping (Fig. 4 and Table 1).

Fig. 4
figure 4

Average Percentage of lettuce not attacked after the test in the various patterns of intercropping (N = 5; p < 0.05). Mean values ± SE followed by different letters are significantly different (ANOVA, LSD test, differences considered significant at P < 0.05)

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Table 1 Univariate tests of significance for average number of lettuce not attacked after the assay sigma-restricted parameterization effective hypothesis decomposition
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Discussion

The lettuce field in monoculture showed a smaller number of plants not attacked after the test from the field in intercropping with onion. Caterpillars threads A. ipsolon intensely attacked the camp in monoculture leaving few plants, this observation could be attributed to the fact that insects use volatile compounds to find the hosts and this phenomenon location in monoculture fields is very easy just because the host would be a released volatile organic compounds. Moreover, it was observed that in areas with mixed cultures (lettuce and onion), few plants were attacked and at the end of study many lettuce plants remained intact. The intercropping with lettuce onion has a direct effect on the caterpillars, A. ipsolon and other insects. This effect can be connected to emission of toxic volatile organic compounds or repellents A. ipsolon also the onion can mask volatile organic compounds released by lettuce used by insects such signals for location [9]. According Mann et al. [10], onion odor emits sulfur containing organic compounds called thiols and these substances can participate in various protective mechanisms described above. Additionally Debra and Misheck [11], obtained similar results to this study, intercropping cabbage crop with onion and garlic, observed that the death of cabbage plants and the incidence of insects significantly reduced while the yield of cabbage was higher. In another study, intercropping was made between wheat and garlic to control the grain aphid Sitobion avenae Fabricius (Homoptera: Aphididae), on the basis of experiments it was observed a reduction in the aphid population and that yields of grain wheat significantly increased [12]. Allium cepa showed to be efficient in control of aphid population while this is intercropped with collard [13].

Several studies on the effect of intercropping in the management of insects have been described in the literature and the results were similar to those found in this work [14–21].

In the same study, tree intercropping patterns were evaluated, and found that the medium surrounds presented a higher number of lettuce plants not attacked in the end of experiment in relation to other patterns. It is important to investigate various forms of intercropping, because depending on how it is made can have an additive effect in reducing the insect pest population. We believe that the border intercropping pattern was the best intercropping pattern of onion with lettuce because, first the caterpillar to enter in the field finds a barrier of onion crop and this attacks the onion, it begins to release a number of volatile organic compounds containing sulfur and these may have a defensive role. This hypothesis is congruent with Huang et al. [22], Dugravot et al. [23], Rouseff et al. [24] and Auger et al. [25], who have investigated the role of volatiles compounds released by Allium spp after damaged and those were classified as toxic or repellents for various insect pest. Mann et al. [10], observed that Sulfur volatiles from Allium spp. affect Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), response to citrus volatiles. This behavior was linked to trisulfides (dimethyl trisulfide) and disulfides and those had inhibition response of D. citri.

Conclusion

Intercropping fits into environmentally acceptable and sustainable vegetable-producing practices. Based on field assays conducted, the intercropping of lettuce with onion resulted in more plant not attacked after the test compared to the monocrop (only lettuce). The  surround (a continuous line encircling onion lettuce line) intercropping pattern was the best intercropping pattern  because, the assay showed a higher average number of non-attacked lettuce plants compared to other patterns. The current results indicate that the onion can be used as a useful tool to control the caterpillar, A. ipsolon in lettuce.