GB2126895A - Compositions and methods to reduce animal browsing damage to plants - Google Patents

Abstract

Damage to plants by browsing animals or insects is reduced by systemically inducing a nonphytotoxic dosage of selenium into the foliage of the plants at a level sufficient to repel animals and insects by means of a selenium compound having a selenium valency not exceeding 4<+>. The method is particularly suited for use in reforestation of pines, firs, and other conifers. The selenium may be systemically absorbed into the plants after application to the surrounding soil in the root zone of seedlings, preferably as a selenium compound, such as sodium or ammonium selenite or dimelamine selenite. Dimelamine selenite is a novel compound.

Description

SPECIFICATION Compositions and methods to reduce animal browsing damage to plants This invention relates to compositions and methods for reducing animal browsing damage to plants by inducing a non-phytotoxic level of selenium in the foliage of the plants. Animals and insects avoid browsing on selenium-doped foliage.

During the development of stands of Douglas fir and other conifers, animals cause the greatest economic damage. Usually the animals browse and clip the stems and foliage of seedlings and saplings; while occasionally root cutting, budding, barking, trampling, and pulling of seedlings occur.

Either the seedlings are killed or their growth is markedly suppressed by the browsing of big game (deer, bear, elk, and the like), hares and rabbits, grouse and other birds, beavers, gophers and other small rodents, domestic stock, and porcupines. Insects may also damage the seedlings or stand.

There are four main ways of avoiding or controlling damage to forest trees and their seedlings by animals: (1) reduce the animal population; (2) exclude the animals from the plants mechanically; (3) repel the animals from browsing; and (4) alter silviculture practices.

Mechanical protection is the most effective, but it is among the most expensive safeguards.

Silviculture practices that may be used include (a) planting resistant species, (b) planting larger seedlings, (c) planting faster growing species, (d) removing and controlling other available food supplies in the plantation which might attract animals, and (e) cutting and replanting in arrangements which deter browsing.

Chemical repellents which affect either odour or taste are of two kinds-systemic and contact. A systemic repellent is applied to the foliage, roots, or soil (in the root zone); is absorbed into the plant; and is translocated to all parts of the plant. A contact repellent is applied to the foliage and stems of plants and remains on the surface of the plants. Two common contact repellents are tetramethylthiuram disulphide and zinc dimethyldithiocarbamate cyclohexamine. When used with conifers, these repellents are usually either sprayed on to the plants at the nursery or the plants are dipped prior to planting. Both are usually applied in 10% concentration in a water solution containing latex adhesives, thickening agents, and defoaming agents. Other contact repellents include a purified fish fraction (PFG), fermented eggs (EV repellent), and human hair.

Systemic repellents offer many advantages over contact repellents, especially in that they provide greater resistance to weathering over longer periods of time. Induced systemic repellency has long been sought, but a safe and effective method has not hitherto been developed.

An example of a natural systemic animal repellent is the occurrence of selenium in certain plants: experiments have shown that animals prefer a selenium-free diet when good forage is available. Plants containing as little as 6-7 FLg/g of selenium have been found to display natural repellency. The selenium-containing plants often act as insect repellents, perhaps by emitting volatile selenium compounds, such as dimethyl selenide or dimethyl diselenide.

The use of selenate compounds (SeO4=) as systemic or contact repellents was investigated by Rediske and Lawrence (Forest Science, Vol. 8, No. 2, 1962, pp. 1 42-148) using Douglas fir seedlings.

They found that sodium selenate applied as a surface coating (5000 ppm Se) was an effective repellent; in fact, it was more effective than the standard tetramethylthiuram disulphide contact repellent. As a systemic repellent, however, selenate was a failure, since a selenium concentration of only 0.5 g/g was toxic to the seedlings, and at this concentration it did not repel animals from browsing. Rediske 8 Lawrence dismissed the possibility of using selenite (SeO3=) in place of selenate on the grounds of its even greater toxicity to plants and its instability.

The present invention is based on the discovery that in fact, when systemic absorption of selenium into plants, particularly forest seedlings, is induced by means of selenium compounds having a selenium valency not exceeding 4+, much higher concentrations of selenium can be tolerated than when selenate is used, up to levels sufficient to repel animals and insects. The invention thus provides a simple, safe, and effective method of inducing systemic animal repellency in plants to reduce browsing damage otherwise caused by animals or to reduce insect damage.

According to the invention, therefore, a method of reducing animal browsing and insect damage to plants comprises inducing a level of selenium systemically in the foliage, by means of a compound of selenium having a selenium valency not exceeding 4+, that is sufficient to repel animals and insects but is substantially non-phytotoxic to the plant.

The method is applicable to all plants which exhibit a tolerance to selenium such that an absorbed amount present in the foliage will reduce browsing yet will not kill a substantial number of plants. It is particularly applicable to reforestation with evergreens, such as Douglas fir, pines, and other conifers. Because the economic loss due to browsing can be great, the method may still be applicable when the dosage level causes ail but the hardiest of plants to die. One-third of the seedlings may be killed, in some circumstances, without the level being excessive. Therefore, the term "nonphytotoxic" should be interpreted liberally to include dosage levels which do kill some plants while allowing hardier plants to grow unimpaired.A selenium concentration in the foliage of at least 10 ,ug/g will generally be required, but it should generally not exceed 400 yglg and is preferably not more than 200 g/g, e.g.

20-200 y9/9. These levels are substantially higher than those that occur naturally in plants.

Applying an effective but nonphytotoxic dosage of selenium by the method of the invention reduces browsing damage by repelling animals, such as deer, elk, bear, and other big game; hares and rabbits, grouse and other birds, beaver; gophers, mice, and other small rodents; domestic stock; and porcupines, without hindering seedling development. Damage by insects, especially to trees of the family Meliaceae, represented by Spanish cedar, can also be substantially reduced.

The selenium may be introduced into the plant by applying the selenium-containing compound to the soil around the roots of the plant so that the roots absorb the selenium and translocate it to the foliage. Selenium compounds that can be used include selenites, e.g. sodium selenite Na2SeO3, ammonium selenite (NH4)2, SeO3, melamine selenite (otherwise known as melamine selenite), and compounds that react when applied to soil to form selenites, selenium dioxide SeO2, ferro-selenium FeSe, and mixtures of these compounds. Selenate compounds should be avoided because they have been found to have a low phytotoxicity level in plants as Douglas fir.

The invention includes dimelamine selenite as a novel compound, and methods of making it.

Dimelamine selenite is particularly useful as it releases selenium progressively with time.

Various aspects of the invention are illustrated and exemplified by the results of numerous tests that are set forth below: Series 1 Absorption of Se from soil Douglas fir seedlings were treated with various levels of selenium in the tetravalent state by applying 1, 10, 100 and 1000 mg doses of SeO2 and Na2SeO3 to the soil of the potted seedlings.

Foliage from the seedlings was collected at various times and was analyzed to determine the selenium content. Table 1 shows the results of the analysis four weeks after applying the selenium compounds.

In each case selenium has moved through the roots of the Douglas fir seedlings into the foliage.

Table 1 Application level Selenium content in Compound (mg) foliage (glg) SeO2 1 1.75 10 44.6 100 398.1 1000 646.2 Na2SeO3 1 39.6 10 41.1 100 177.9 1000 3446.3 Series 2 Absorption of Se from solution Douglas fir seedlings were cultured in Hoaglund's solution containing respectively 10, 20, 40 and 80 mg of Na2SeO3 in two litres of solution. In all the seedlings, the concentration of selenium in the foliage increased progressively at a rate which declined as equilibrium was approached.

Series 3 Effect of cation on Se absorption from selenite Douglas fir seedlings were treated individually using solutions which contained 10 mg. of Se as sodium and ammonium selenite respectively. The concentration of Se in the foliage after 1,2 and 3 weeks is shown in Table 2. The results show that selenium is initially absorbed faster if supplied as Na2SeO3 than as (NH4)2SeO3, perhaps owing to the difference in the basicity of NaOH and NH4OH.

However, the difference in the absolute amounts of Se absorbed tends to disappear as treatment continues.

Table 2 Selenium content in foliage (jugig) Time (weeks) (NH2SeO3* Na2SeO3* 1 0.94 6.5 2 8.6 13.9 3 12.4 14.3 *Application level 10 mg (selenium basis).

Series 4 Distribution of Se in the plant The roots, stem and branches of Douglas fir seedlings into which Se had been introduced by the method of the invention were analysed separately. The roots always had the highest level, usually well above that of the stem, branches, and foliage. All parts show an increase in selenium content upon treatment. The level in the stem, branches, and foliage is relatively equal; if significant, the foliage usually has the lowest amount of selenium.

Series 5 Phytotoxic effect Douglas fir seedlings were treated with various amounts of SeO2 or Na2SeO3 in pot and solution culture and their growth was observed. The results are set forth in Table 3.

Table 3 Selenium Foliage application content Compound level (mg) 4tg/g) Observation SeO2 1 (pot) 18 no damage 10 ,, 45 no damage 100 ,, 398 1/3 dead 1000 ,, 646 all dead Na2SeO3 1 (pot) 40 no damage 10 ,, 41 no damage 100 ,, 178 1/3 dead 1000 ,, 3446 all dead Na2SeO3 10 (solution) 104 no damage 20 ,, 314 no damage 40 ,, 575 dried foliage* 60 ,, 692 dried foliage* 80 " 832 dried foliage* (*indicates the first stages of death in the plant.) The phytotoxic level is difficult to determine from these results.While one-third of the seedlings died at a foliage content of 1 78 yg/g when Na2SeO3 was applied to the soil, no damage occurred when the level reached 314 yg/g in solution culture and only one-third were killed at a level of 398 Xug/g when SeO2 was applied to the soil. For reforestation purposes, a level of less than about 400 jug/g in the foliage probably will be useable without undue damage to the seedlings (even though some deaths are induced). Levels below about 200 tlg/g are preferred.

The results of the preceding tests show that absorption of selenium depends both on the compound applied and the conditions in which the selenium is applied. In solution culture, the seedlings absorbed much more selenium for the same dosage level than when selenium was applied to the soil. Soils differ in composition; it is likely that seedlings in these different soils will display different absorption characteristics.

Series 6 Repellency to animal browsing (a) In the laboratory, Douglas fir foliage selenised in accordance with the invention, and similar non-selenised foliage, was placed in the cages of rabbits. The selenised foliage contained between 10 and 20 ,ug/g of selenium. Consumption of selenised foliage was less than that of the non-selenised.

(b) In field tests, Douglas fir seedlings were selenised by applying 40 mg Se in solution to the soil around each seedling as Na2SeO3. In two plantations no browsing damage to the treated seedlings occurred throughout a four-week study, although deer were .observed in the plantation and browsing did occur on seedlings which were untreated.

Series 7 Other conifers Ponderosa pine, white pine, and Western red cedar were treated with various levels of SeO2 by application to the soil around seedlings. Foliage was collected and analysed to determine the selenium content. Tables 4 and 5 show the results of these tests. Selenium was systemically absorbed into the seedlings and moved through the roots into the foliage.

Table 4 Application Selenium content Species level (mug) in foliage {yg/g) Ponderosa pine 0 0 10 0 20 2 100 5 500 50 White pine 0 0 10 4 20 4 100 50 500 485 Western red cedar 0 0 10 5 20 4 100 9 500 119 Table 5 Survival rate (%) of conifer seedlings one week after application of various amounts of selenium dioxide Level of application, mg Se/seedling Species 0 10 20 100 500 Ponderosa pine 100% 100% 100% 75% 50% White pine 100% 100% 100% 0% 0% Western red cedar 100% 75% 100% 0% 0% Series 8 Variation with time Field tests showed that the level of selenium in the foliage of Douglas fir seedlings treated according to the invention fluctuated during the period following application of the selenium compound, reaching a peak and then declining.This occurs because selenium is diffused from the foliage in volatile compounds, such as dimethyl selenide. Also, the level of selenium in the soil decreases because the selenium is leached away from the plant's root zone by rain and is converted to nonabsorbable forms of selenium by biological action.

To make the dosage level more uniform with time, the selenium may be supplied as a compound that progressively releases it in an absorbable form. One such compound is ferroselenium, which is an inexpensive, high selenium content powder which is water insoluble. The ferroselenium is nonabsorbable in its selenide form; through chemical conversion in the soil, however, the ferroselenium provides absorbable selenium compounds.

Care must however be taken when using FeSe, as damage was observed to occur in the seedlings, probably because the selenium was oxidised to both selenite (SeO3=) and selenate (SeO4), which is toxic to Douglas fir at low dosages (about 0.5 g/g).

Series 9 Synthesis of melamine selenite The novel compound dimelamine selenite may be prepared by the reaction of selenium dioxide, either as an aqueous solution or as solid, with an aqueous (preferably saturated) solution of melamine.

By way of example, 20 g (a half-molar equivalent) of melamine were dissolved in 400 ml distilled water and heated with stirring to 900C; 8.7978 g of selenium dioxide were dissolved in 100 ml distilled water and added to the melamine solution; both heating and stirring were immediately discontinued; the solution was allowed to cool to room temperature, and then was cooled to 50C by contacting with an ice bath and the white crystaliine needles which formed were vacuum filtered and weighed, with a yield of 27.0313 g.

A melting point determination was attempted, but the compound decomposed at 2700C+20C to a pink colour characteristic of elemental selenium. The solubility of the compound was determined to be 1.58 g salt per 100 g aqueous solution at 200 C. Such a solution had a pH of 5.3. An elemental analysis of the salt was carried out with the following results (standard deviations are indicated by +): elemental compositions of both doubly and triply hydrated dimelamine selenite fail within the uncertainty limits of this analysis. The density of the salt has been estimated gravimetrically to be 1.315 g/cm3.

The 2-3 moles of water appear to be quite tightly bound as they remain upon recrystallisation.

Demelamine selenite (prepared as above) in 1 50, 750, and 1 500 mg amounts was mixed with the upper surface of the soil of potted Spanish cedars. The trees were watered once a week and kept in a greenhouse. Tree heights at treatment time were approximately 80-1 00 cm. Foliage samples were taken after 5 months and the selenium levels were determined: 1 50 mg 6.6 ppm Se, 750 mg67.8 ppm Se, and 1 500 mg-29 ppm Se. The tree which received 1 500 mg dimelamine selenite began exhibiting a pronounced yellow discolouration of its foliage within two months after treatment and appeared to be nearly dead at the time of sample collection. The other two trees did not appear to be adversely affected by the selenite salt.

Series 10 Insect repellency Application of dimelamine selenite to the soil around a plant, such as Spanish cedar (Cedrela Odorata), and subsequently absorption and translocation of the selenium to the foliage repels boring or sucking insects (or kills them). Spanish cedar, of the family Meliaceae, may be protected from attack by the shootborer (Hypsipyla grandella or Hypsipyla robusta) by selenising the foliage according to the method of this invention. These trees are fast-growing, lumber-producing trees of the cedar and mahogany type.

Claims (12)

Claims

1. A method in which a level of selenium is systemically induced in the foliage of a plant, by means of a compound of selenium having a selenium valency not exceeding 4+, that is sufficient to repel animals and insects but is substantially non-phytotoxic to the plant.

2. A method according to claim 1 applied to a seedling of an evergreen.

3. A method according to claim 1 applied to a seedling of a conifer.

4. A method according to claim 1 applied to a Douglas fir seedling.

5. A method according to claim 1 applied to a seedling of the family Meliaceae.

6. A method according to any preceding claim in which the compound of selenium is applied to the soil around a seedling of the plant.

7. A method according to claim 6 in which the compound of selenium is a selenite or reacts with the soil to form a selenite.

8. A method according to claim 7 in which the compound of selenium is sodium selenite or ammonium selenite.

9. A method according to claim 7 in which the compound of selenium is dimelamine selenite.

10. A method according to any preceding claim in which the level of selenium induced in the foliage is at least 10 FLg/g.

11. A method according to claim 10 in which the level of selenium induced in the foliage does not exceed 200 g/g.

12. Dimelamine selenite.

1 3. A method in which dimelamine selenite is produced by the reaction of selenium dioxide with an aqueous solution of melamine.